feat!: drop node8 support, support for async iterators

[alexander-fenster]

BREAKING CHANGE: The library now supports Node.js v10+. The last version to support Node.js v8 is tagged legacy-8 on NPM.

New feature: methods with pagination now support async iteration.

[codecov]

Codecov Report

Merging #364 into master will increase coverage by 8.38%.
The diff coverage is 99.42%.

@@            Coverage Diff             @@
##           master     #364      +/-   ##
==========================================
+ Coverage   89.71%   98.09%   +8.38%     
==========================================
  Files           7        7              
  Lines        8011     8938     +927     
  Branches      253      335      +82     
==========================================
+ Hits         7187     8768    +1581     
+ Misses        820      166     -654     
  Partials        4        4              

Impacted Files	Coverage Δ
src/v1beta1/auto_ml_client.ts	98.37% <ø> (+7.26%)	⬆️
src/v1/auto_ml_client.ts	98.21% <99.24%> (+6.62%)	⬆️
src/v1/prediction_service_client.ts	97.16% <100.00%> (+10.70%)	⬆️
src/v1beta1/prediction_service_client.ts	97.46% <100.00%> (+14.57%)	⬆️

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