feat: add support for StageVariables

[keetonian]

Issue #, if available:
N/A

Description of changes:
Adds support for StageVariables for HttpApis.

Description of how you validated changes:
Deployed to CFN.

Checklist:

• Write/update tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected
• Add/update example to examples/2016-10-31

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #1493 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1493   +/-   ##
========================================
  Coverage    94.13%   94.13%           
========================================
  Files           78       78           
  Lines         4926     4926           
  Branches       991      991           
========================================
  Hits          4637     4637           
  Misses         134      134           
  Partials       155      155

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