feat: turned google-proto-files into a dev dependency

[hiranya911]

Fixes #469

This PR changes the dependency on google-proto-files into a dev dependency, and adds a script which bundles the required proto files into google-gax itself. This increases the size of the google-gax by about 750KB, but saves users about 10MB in the process. Appreciate your feedback on this solution.

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #470 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #470   +/-   ##
=======================================
  Coverage   88.88%   88.88%           
=======================================
  Files           1        1           
  Lines         333      333           
=======================================
  Hits          296      296           
  Misses         37       37

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[alexander-fenster]

Thank you for doing this! I'm OK with the logic of this change, the only concern I have is that we don't have a real consumer test for gax (i.e. the test that will npm pack the package and verify it can be consumed correctly, checking all the features including those requiring the extra protos such as long running operations).

I'm working on adding such a test this week so if you don't mind let's hold this PR until such test is there (hopefully today or tomorrow) and then we'll be able to make this change being confident that nothing is broken.

[JustinBeckwith]

Marking this as blocked on #471

[alexander-fenster]

Thank you for the updates!