Configurable SnippetGen: add return type to the sample function

The sample function's return type is specified in the snippet config.

This and the sample function parameters (https://github.com/googleapis/google-cloud-python/issues/16419) will require the functionality of translating recursively defined `snippet_config_language.Type` into Python's type annotation (in libcst: https://libcst.readthedocs.io/en/latest/nodes.html#libcst.FunctionDef.returns).