feat: node-http-handler set default keep-alive to true

[trivikr]

Issue #, if available:
aws/aws-sdk-js#2571

Description of changes:

• added keepAlive in NodeHttpOptions
• modified node-http-handler to accept keepAlive
• set default keepAlive to true
• ran node scripts/rebuildClients.ts to update node clients

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-io]

Codecov Report

Merging #307 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
+ Coverage   91.35%   91.37%   +0.02%     
==========================================
  Files         266      266              
  Lines        6081     6097      +16     
  Branches     1364     1368       +4     
==========================================
+ Hits         5555     5571      +16     
  Misses        526      526

Impacted Files	Coverage Δ
...iceCustomizations/customizationsFromModel/index.ts	100% <ø> (ø)	⬆️
...ackages/node-http-handler/src/node-http-handler.ts	92.72% <100%> (+0.89%)	⬆️
...omizationsFromModel/httpConfigurationProperties.ts	100% <100%> (ø)	⬆️

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