feat: Add API stage tags support (#384)

[eduardovra]

Issue #, if available:
#384

Description of changes:
Add support to tags configuration on API stages.

Description of how you validated changes:
I confirmed that the transformed template is successfully deployed and the tags are created correctly.

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

PS. I wasn't sure if this feature required to be added in the examples referred by the checklist. Please let me know if this is necessary and I'll add this in the PR.

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 #993 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #993      +/-   ##
===========================================
+ Coverage    94.71%   94.72%   +<.01%     
===========================================
  Files           69       69              
  Lines         3160     3164       +4     
  Branches       606      607       +1     
===========================================
+ Hits          2993     2997       +4     
  Misses          85       85              
  Partials        82       82

Impacted Files	Coverage Δ
samtranslator/model/sam_resources.py	95.59% <ø> (ø)	⬆️
samtranslator/model/apigateway.py	97.72% <100%> (ø)	⬆️
samtranslator/model/api/api_generator.py	93.68% <100%> (+0.09%)	⬆️

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[jlhood]

Excellent PR! I see this is your first contribution to SAM. Thanks so much. We really appreciate the help!