feat: Deprecate LDUser related functionality (#238)

LDUser functionality has been replaced with the more flexible LDContext
concept which all LaunchDarkly SDKs will support.

We retained LDUser through the first major release to aide with the
adoption transition, but as we near the next release, we need to alert
customers that support will be ending.

LaunchDarkly/LaunchDarkly/LDClient.swift

@@ -291,6 +291,7 @@ public class LDClient {
 
      This LDUser will be converted into an LDContext, and the context specific version of this method will be called. See `identify(context:completion:)` for details.
      */
+    @available(*, deprecated, message: "Use identify(context:completion:)")
     public func identify(user: LDUser, completion: (() -> Void)? = nil) {
         switch user.toContext() {
         case .failure(let error):
@@ -601,6 +602,7 @@ public class LDClient {
 
      This LDUser will be converted into an LDContext, and the context specific version of this method will be called. See `start(config:context:completion:)` for details.
      */
+    @available(*, deprecated, message: "Use start(config:context:completion:)")
     public static func start(config: LDConfig, user: LDUser? = nil, completion: (() -> Void)? = nil) {
         switch user?.toContext() {
         case nil:
@@ -664,6 +666,7 @@ public class LDClient {
 
      This LDUser will be converted into an LDContext, and the context specific version of this method will be called. See `start(config:context:startWaitSeconds:completion:)` for details.
      */
+    @available(*, deprecated, message: "Use start(config:context:startWithSeconds:completion:)")
     public static func start(config: LDConfig, user: LDUser? = nil, startWaitSeconds: TimeInterval, completion: ((_ timedOut: Bool) -> Void)? = nil) {
         switch user?.toContext() {
         case nil:

LaunchDarkly/LaunchDarkly/Models/LDUser.swift

@@ -6,6 +6,7 @@ import Foundation
  The usage of LDUser is no longer recommended and is retained only to ease the adoption of the `LDContext` class. New
  code using this SDK should make use of the `LDContextBuilder` to construct an equivalent `Kind.user` kind context.
  */
+@available(*, deprecated, message: "Use LDContextBuilder to construct a context of kind 'user'")
 public struct LDUser: Encodable, Equatable {
 
     static let optionalAttributes = UserAttribute.BuiltIn.allBuiltIns.filter { $0.name != "key" && $0.name != "anonymous"}
@@ -217,6 +218,7 @@ public struct LDUser: Encodable, Equatable {
 }
 
 /// Class providing ObjC interoperability with the LDUser struct
+@available(*, deprecated)
 @objc final class LDUserWrapper: NSObject {
     let wrapped: LDUser
 
@@ -226,4 +228,5 @@ public struct LDUser: Encodable, Equatable {
     }
 }
 
+@available(*, deprecated)
 extension LDUser: TypeIdentifying { }

LaunchDarkly/LaunchDarkly/Models/UserAttribute.swift

@@ -9,6 +9,7 @@ import Foundation
  reference guides [Setting user attributes](https://docs.launchdarkly.com/home/users/attributes) and
  [Targeting users](https://docs.launchdarkly.com/home/flags/targeting-users).
  */
+@available(*, deprecated)
 public class UserAttribute: Equatable, Hashable {
 
     /**

LaunchDarkly/LaunchDarkly/ObjectiveC/ObjcLDClient.swift

@@ -119,6 +119,7 @@ public final class ObjcLDClient: NSObject {
 
         This LDUser will be converted into an LDContext, and the context specific version of this method will be called. See `identify(context)` for details.
      */
+    @available(*, deprecated, message: "Use identify(context)")
     @objc public func identify(user: ObjcLDUser) {
         ldClient.identify(user: user.user, completion: nil)
     }
@@ -144,6 +145,7 @@ public final class ObjcLDClient: NSObject {
 
         This LDUser will be converted into an LDContext, and the context specific version of this method will be called. See `identify(context:completion:)` for details.
      */
+    @available(*, deprecated, message: "Use identify(context:completion:)")
     @objc public func identify(user: ObjcLDUser, completion: (() -> Void)? = nil) {
         ldClient.identify(user: user.user, completion: completion)
     }
@@ -577,6 +579,7 @@ public final class ObjcLDClient: NSObject {
 
         This LDUser will be converted into an LDContext, and the context specific version of this method will be called. See `start(configuration:context:completion:)` for details.
      */
+    @available(*, deprecated, message: "Use start(configuration:context:completion:)")
     @objc public static func start(configuration: ObjcLDConfig, user: ObjcLDUser, completion: (() -> Void)? = nil) {
        LDClient.start(config: configuration.config, user: user.user, completion: completion)
    }
@@ -598,6 +601,7 @@ public final class ObjcLDClient: NSObject {
 
         This LDUser will be converted into an LDContext, and the context specific version of this method will be called. See `start(configuration:context:startWaitSeconds:completion:)` for details.
      */
+   @available(*, deprecated, message: "Use start(configuration:context:startWaitSeconds:completion:)")
    @objc public static func start(configuration: ObjcLDConfig, user: ObjcLDUser, startWaitSeconds: TimeInterval, completion: ((_ timedOut: Bool) -> Void)? = nil) {
        LDClient.start(config: configuration.config, user: user.user, startWaitSeconds: startWaitSeconds, completion: completion)
    }

LaunchDarkly/LaunchDarkly/ObjectiveC/ObjcLDUser.swift

@@ -7,6 +7,7 @@ import Foundation
  code using this SDK should make use of the `LDContextBuilder` to construct an equivalent `Kind.user` kind context.
  */
 @objc (LDUser)
+@available(*, deprecated, message: "Use ObjcLDContextBuilder to construct a context of kind 'user'")
 public final class ObjcLDUser: NSObject {
     var user: LDUser