Add 'webapp2' alias for 'google.appengine.ext.webapp'.

Follow up to #1121.

**Main changes:**
* When snippets are generated, snippet metadata is also generated.
* If a method has a snippet, that snippet is included in the method docstring.

**Other changes:**
* Removed the method docstring in the code snippet file (line right below the method definition) since the same text is already in the comment block at the top of the file.
* Removed the concept of a "standalone" sample. All generated samples are expected to be standalone. When someone wants a smaller portion of the sample (e.g., request initialization only) they should fetch it from the file by looking up the line numbers in the snippet metadata file.

Other Notes:
* ~It doesn't look like it's possible to do type annotations with `_pb2` types, so those are annotated as `Any`.~ It is possible to do mypy checking with https://github.com/dropbox/mypy-protobuf, but I think it will be easier make that change in a separate PR.
* There are a lot of golden file updates, [this range of commits](https://github.com/googleapis/gapic-generator-python/pull/1129/files/872c156f5100f1de20631dd59d083206432db374) has _most_ of the generator and test changes.