Fix for broken relative imports and missing betterproto_lib_google_protobuf

The issue affects both relative imports and google.protobuf imports, but is not 100% reproducible.

My working theory is that for service method input types `output_package.imports` sometimes populates too late to be included in the imports section of a generated file. The list is populated only when template is compiled with `method.py_input_message_param` which comes way after the import section.

On the contrary, imports for output types are figured out before template compilation in `ServiceMethodCompiler:__post__init__` by using `self.py_output_message_type`

Fix is to add
```
if "Optional" in self.py_input_message_type:
self.output_file.typing_imports.add("Optional")
```

I have only checked it with google.protobuf types, but I suspect that relative imports behave no different and this issue is related to #441