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 #299 into master will increase coverage by 5.02%.
The diff coverage is 99.11%.

@@            Coverage Diff             @@
##           master     #299      +/-   ##
==========================================
+ Coverage   91.22%   96.24%   +5.02%     
==========================================
  Files          11       11              
  Lines        3782     4045     +263     
  Branches      203      230      +27     
==========================================
+ Hits         3450     3893     +443     
+ Misses        327      147     -180     
  Partials        5        5              

Impacted Files	Coverage Δ
src/v1p1beta1/asset_service_client.ts	95.53% <96.87%> (+3.87%)	⬆️
src/v1/asset_service_client.ts	97.18% <100.00%> (+5.00%)	⬆️
src/v1beta1/asset_service_client.ts	95.24% <100.00%> (+3.74%)	⬆️
src/v1p2beta1/asset_service_client.ts	96.55% <100.00%> (+8.33%)	⬆️
src/v1p4beta1/asset_service_client.ts	94.92% <100.00%> (+3.98%)	⬆️

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