Add public factory methods for CallResult.

[mpetrov]

This enables instantiating CallResult structs for test stubs.

[fandyfyf]

👍

[MrMage]

Looks good to me, but would you mind providing a short example illustrating when these are needed to write tests?

[mpetrov]

Thanks Daniel!

This could be used for stubbing out the async calls as follows:

  override func get(_ request: Echo_EchoRequest, metadata customMetadata: Metadata, completion: @escaping (Echo_EchoResponse?, CallResult) -> Void) throws -> Echo_EchoGetCall {
    completion(nil, .error(statusCode: .notFound))

The bazel rules don't currently enable stub generation, but once that's in place I'll likely also change the default stub code as well.

[MrMage]

Makes sense, thank you for the elaboration!