[ObjectSchema] Expose objectClass from RLMObjectSchema private interface

[liamnichols]

Summary

This change proposes the addition of the computed variable public var objectClass: AnyClass { get } on ObjectSchema to expose the underlying RLMObjectSchema's private objectClass.

Motivation

When working with Swift models, realm-cocoa will 'demangle' the original class name when building the shared schema. As a result, no mechanism exists for looking up the associated class type from a Realms schema. This isn't something that most people will need to do, but it can be really beneficial in cases where we might need to store some additional information on model definitions that needs to be validated either at runtime or in unit tests.

Although the objectClass is private on RLMObjectSchema, a similar capability is possible in projects that use realm-cocoa directly since they'd be able to use NSClassFromString(objectSchema.className) however this will return nil in Swift using objectSchema.className since the value has been passed through +[RLMSwiftSupport demangleClassName:]

I hope there is no problem with such change, my inspiration for the change was that I noticed Realm.Configuration is actually doing a similar thing in order to expose the objectTypes getter since they're backed by a custom RLMSchema but I wanted similar functionality when reading object schemas from a Realm directly

[tgoyne]

I need to think about this for a bit. There isn't any obvious reason why it'd be a problem (even the edge cases like inspecting the schema during migrations are probably fine), but it is the first thing that ties a Swift schema type's public interface to a specific Realm.

[realm-probot]

Hey - looks like you forgot to add a T-* label - could you please add one (if you have access to add labels)?

[tgoyne]

I failed to come up with any plausible way that this could be a problem that doesn't involve us making breaking changes anyway, so seems fine.

[TTLoveQX]

Hello there is a crash on realm. It crashed 'libc++abi.dylib: terminating with uncaught exception of type NSException' when I run it on Xcode 11.