Generate a @kotlin.Deprecated annotation for deprecated RPCs

[timostamm]

Protobuf has a built-in option to mark elements as deprecated. For example:

syntax="proto3";

service FooService {
  rpc Foo(FooRequest) returns (FooResponse) {
    // Is this method deprecated?
    // Depending on the target platform, this can emit Deprecated annotations
    // for the method, or it will be completely ignored; in the very least,
    // this is a formalization for deprecating methods.
    option deprecated = true;
  }
}

message FooRequest {}
message FooResponse {}

The gRPC implementation generates a kotlin.Deprecated annotation for this RPC:

    @Deprecated("The underlying service method is marked deprecated.")
    public suspend fun foo(request: FooRequest, headers: Metadata = Metadata()): FooResponse = ...

The feature was added in grpc/grpc-kotlin#264

Since it can be useful feature for team collaboration, I think we should do the same in the plugin for Connect.

For the example above, the generated client would change as follows:

public class FooServiceClient(
  private val client: ProtocolClientInterface,
) : FooServiceClientInterface {
+ @kotlin.Deprecated("The underlying service method is marked deprecated.")
  override suspend fun foo(request: FooRequest, headers: Headers): ResponseMessage<FooResponse> =

The generated client interface would change as well:

public interface FooServiceClientInterface {
+ @kotlin.Deprecated("The underlying service method is marked deprecated.")
  public suspend fun foo(request: FooRequest, headers: Headers = emptyMap()):