refactor: drop dependencies on google-proto-files and async

[JustinBeckwith]

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@e0b4518). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #329   +/-   ##
=========================================
  Coverage          ?   96.39%           
=========================================
  Files             ?       13           
  Lines             ?     1053           
  Branches          ?      143           
=========================================
  Hits              ?     1015           
  Misses            ?       32           
  Partials          ?        6

Impacted Files	Coverage Δ
src/subscription.ts	99.06% <ø> (ø)
src/topic.ts	98.64% <ø> (ø)
src/publisher.ts	98.03% <ø> (ø)

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