feat: allow pollIntervalMs to be configured

[bcoe]

Allows pollIntervalMs to be configured for long running operations, when instantiating a Compute instance:

new Compute({
  pollIntervalMs: 2000
})

[codecov]

Codecov Report

Merging #392 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #392      +/-   ##
=========================================
+ Coverage    99.8%   99.8%   +<.01%     
=========================================
  Files          20      20              
  Lines       11712   11739      +27     
=========================================
+ Hits        11689   11716      +27     
  Misses         23      23

Impacted Files	Coverage Δ
src/instance-group.js	100% <100%> (ø)	⬆️
src/autoscaler.js	100% <100%> (ø)	⬆️
src/network.js	100% <100%> (ø)	⬆️
src/snapshot.js	100% <100%> (ø)	⬆️
src/rule.js	100% <100%> (ø)	⬆️
src/index.js	100% <100%> (ø)	⬆️
src/firewall.js	100% <100%> (ø)	⬆️
src/zone.js	100% <100%> (ø)	⬆️
src/instance-group-manager.js	100% <100%> (ø)	⬆️
src/subnetwork.js	100% <100%> (ø)	⬆️
... and 10 more

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