Add Schema field to Spec for introspection

[emcfarlane]

New field Schema of type any on Spec. For proto based schemas the type will be of protoreflect.MethodDescriptor. This allows for easy introspection to interceptors.

Updated generated code to add the new WithSchema option to both handlers and clients. Schema is fetched from the proto file using the file.GoDescriptorIdent. To ensure generated code is matched with a library that includes the option a new version tag is added for v1.13.

Example

For a unary request one can access the input and output descriptors by casting the interface and using protoreflect:

methodDesc, ok := req.Spec().Schema.(protoreflect.MethodDescriptor)
if !ok {
	return nil, fmt.Errorf("expected protoreflect.MethodDescriptor got %T", req.Spec().Schema)
}
inputDesc := methodDesc.Input()
outputDesc := methodDesc.Output()

Fixes #571 and #600