Add generateCoroutineMethods plugin option for generating unary callback signatures

[buildbreaker]

Adding the callbackSignature option for call sites without kotlin coroutines. This change basically exposes the wiring from the generated code to the underlying unary request callback method.

Option	Type	Default	Repeatable	Details
generateCallbackMethods	Boolean	false	No	Generate callback signatures for unary methods.
generateCoroutineMethods	Boolean	true	No	Generate suspend signatures for unary methods.

Interface example:

public interface ElizaServiceClientInterface {
    public suspend fun say(request: NoPackage.SayRequest, headers: Headers = emptyMap()):
        ResponseMessage<NoPackage.SayResponse>

    public fun say(
        request: NoPackage.SayRequest,
        headers: Headers = emptyMap(),
        onResult: (ResponseMessage<NoPackage.SayResponse>) -> Unit
    ): Cancelable
}

Implementation:

public class ElizaServiceClient(
    private val client: ProtocolClientInterface
) : ElizaServiceClientInterface {
    public override suspend fun say(request: NoPackage.SayRequest, headers: Headers):
        ResponseMessage<NoPackage.SayResponse> = client.unary(
        request,
        headers,
        MethodSpec(
            ".ElizaService/Say",
            NoPackage.SayRequest::class,
            NoPackage.SayResponse::class
        )
    )

    public override fun say(
        request: NoPackage.SayRequest,
        headers: Headers,
        onResult: (ResponseMessage<NoPackage.SayResponse>) -> Unit
    ): Cancelable = client.unary(
        request,
        headers,
        MethodSpec(
            ".ElizaService/Say",
            NoPackage.SayRequest::class,
            NoPackage.SayResponse::class
        ),
        onResult
    )
}

[rebello95]

1. Would it make sense to duplicate crosstests for callbacks vs coroutines (we do in connect-swift)?
2. Should we add an option to turn off the coroutine API generation (like we have for Swift async/await) so that users don't unnecessarily generate unused code?