Build services with a derived configuration object

This is a pared-down version of #2809.

This is the code-generated version of the
david-perez/smithy-rs-service-config#2 POC.

This introduces a code-generated `Config` object which is provided to
the service builder constructor via `PokemonService::builder(config:
Config)`. This will displace the current `builder_without_plugins` and
`builder_with_plugins`, deprecating them. We will eventually remove
them.

The motivation is to have a single place where to register plugins and
layers. Smithy traits can leverage this object to inject methods that
can apply plugins and layers. For example, an `@authorization` trait
implementer could vend a smithy-rs decorator that pre-applied an
authorization plugin inside the `Config` object, possibly exposing a
method to pass in any runtime arguments needed to configure the
middleware.

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustWriter.kt

@@ -551,10 +551,7 @@ class RustWriter private constructor(
         if (this.className.contains("AbstractCodeWriter") || this.className.startsWith("java.lang")) {
             return false
         }
-        if (this.fileName == "RustWriter.kt") {
-            return false
-        }
-        return true
+        return this.fileName != "RustWriter.kt"
     }
 
     private val preamble = mutableListOf<Writable>()

codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ServerCodegenVisitor.kt

@@ -73,6 +73,7 @@ import software.amazon.smithy.rust.codegen.server.smithy.generators.ServerRootGe
 import software.amazon.smithy.rust.codegen.server.smithy.generators.ServerRuntimeTypesReExportsGenerator
 import software.amazon.smithy.rust.codegen.server.smithy.generators.ServerServiceGenerator
 import software.amazon.smithy.rust.codegen.server.smithy.generators.ServerStructureConstrainedTraitImpl
+import software.amazon.smithy.rust.codegen.server.smithy.generators.ServiceConfigGenerator
 import software.amazon.smithy.rust.codegen.server.smithy.generators.UnconstrainedCollectionGenerator
 import software.amazon.smithy.rust.codegen.server.smithy.generators.UnconstrainedMapGenerator
 import software.amazon.smithy.rust.codegen.server.smithy.generators.UnconstrainedUnionGenerator
@@ -613,7 +614,10 @@ open class ServerCodegenVisitor(
                 serverProtocol,
             ).render(this)
 
+            ServiceConfigGenerator(codegenContext).render(this)
+
             ScopeMacroGenerator(codegenContext).render(this)
+
         }
     }
 

codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/generators/ServerRootGenerator.kt

@@ -121,7 +121,7 @@ open class ServerRootGenerator(
             //! let http_plugins = HttpPlugins::new()
             //!         .push(LoggingPlugin)
             //!         .push(MetricsPlugin);
-            //! let builder: $builderName<Body, _, _> = $serviceName::builder_with_plugins(http_plugins, IdentityPlugin);
+            //! let builder: $builderName<Body, _, _, _> = $serviceName::builder_with_plugins(http_plugins, IdentityPlugin);
             //! ```
             //!
             //! Check out [`#{SmithyHttpServer}::plugin`] to learn more about plugins.
@@ -237,6 +237,6 @@ open class ServerRootGenerator(
     fun render(rustWriter: RustWriter) {
         documentation(rustWriter)
 
-        rustWriter.rust("pub use crate::service::{$serviceName, ${serviceName}Builder, MissingOperationsError};")
+        rustWriter.rust("pub use crate::service::{$serviceName, ${serviceName}Builder, Config, MissingOperationsError};")
     }
 }