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 #396 into master will increase coverage by 21.39%.
The diff coverage is 98.8%.

@@             Coverage Diff             @@
##           master     #396       +/-   ##
===========================================
+ Coverage   77.16%   98.55%   +21.39%     
===========================================
  Files           8        8               
  Lines       14912    16040     +1128     
  Branches      323      392       +69     
===========================================
+ Hits        11507    15809     +4302     
+ Misses       3399      225     -3174     
  Partials        6        6

Impacted Files	Coverage Δ
src/v3/index.ts	100% <100%> (ø)	⬆️
src/v3/uptime_check_service_client.ts	98.45% <98.63%> (+22.51%)	⬆️
src/v3/group_service_client.ts	98.49% <98.65%> (+22.16%)	⬆️
src/v3/metric_service_client.ts	98.66% <98.7%> (+22.06%)	⬆️
src/v3/alert_policy_service_client.ts	98.51% <98.86%> (+23.64%)	⬆️
src/v3/service_monitoring_service_client.ts	98.54% <98.92%> (+20.03%)	⬆️
src/v3/notification_channel_service_client.ts	98.6% <98.99%> (+19.2%)	⬆️
... and 3 more

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[AVaksman]

@googlebot I consent.