feat: implement typed function signature

[garethgeorge]

Provide a strongly typed function signature for the ruby functions framework. The strongly typed function signature will provide JSON request deserialization and JSON response serialization by default. It may additionally be used with types implementing decode_json and to_json.

[dazuma]

This is looking much nicer now. Can you also add typed to the test framework (lib/functions_framework/testing.rb)? You can pretty much just copy call_http and test for :typed rather than :http and change a few names and yardoc comments.

[garethgeorge]

Thanks for the review @dazuma! Added support for the typed signature in the testing package.