fix(types): extend constructor options from gax

[stephenplusplus]

This looks more correct to me, but please review!

gax.GoogleAuthOptions is a re-export of require('google-auth-library').GoogleAuthOptions.

gax.GrpcClientOptions extends GoogleAuthOptions with these properties: auth, promise, and grpc.

[codecov]

Codecov Report

Merging #676 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #676      +/-   ##
==========================================
- Coverage   68.25%   68.23%   -0.02%     
==========================================
  Files          36       36              
  Lines       10599    10599              
  Branches      174      174              
==========================================
- Hits         7234     7232       -2     
- Misses       3364     3366       +2     
  Partials        1        1

Impacted Files	Coverage Δ
src/index.ts	99.65% <100%> (ø)	⬆️
src/sink.ts	98.3% <0%> (-0.57%)	⬇️

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