validate Http Api Authorizers identity Header

[jonife]

Issue #, if available:

Description of changes:
added validation to validate the property type of Authorizers Identity parameter Headers to arrest a situation where an intrinsic value can't be resolved.

Description of how you validated changes:
created a test sam template and tested the exception

Checklist:

• Add/update unit tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• Add/update integration tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/aws-sam-cli-app-templates/

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

[hoffa]

To prevent the duplication:

Suggested change

	if self.identity.get("Headers"):
	headers = self.identity.get("Headers")
	headers = self.identity.get("Headers")
	if headers:

[codecov-commenter]

Codecov Report

Merging #2339 (b0f45e4) into develop (e7a1496) will increase coverage by 0.80%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2339      +/-   ##
===========================================
+ Coverage    93.58%   94.38%   +0.80%     
===========================================
  Files           90       98       +8     
  Lines         6124     7217    +1093     
  Branches      1260     1493     +233     
===========================================
+ Hits          5731     6812    +1081     
- Misses         183      196      +13     
+ Partials       210      209       -1     

Impacted Files	Coverage Δ
samtranslator/region_configuration.py	77.77% <0.00%> (-22.23%)	⬇️
samtranslator/validator/validator.py	91.80% <0.00%> (-3.85%)	⬇️
samtranslator/model/exceptions.py	97.67% <0.00%> (-2.33%)	⬇️
samtranslator/open_api/open_api.py	90.03% <0.00%> (-1.94%)	⬇️
samtranslator/model/s3_utils/uri_parser.py	68.42% <0.00%> (-0.81%)	⬇️
samtranslator/yaml_helper.py	89.47% <0.00%> (-0.53%)	⬇️
samtranslator/translator/logical_id_generator.py	90.62% <0.00%> (-0.29%)	⬇️
samtranslator/model/api/api_generator.py	94.08% <0.00%> (-0.28%)	⬇️
samtranslator/model/apigateway.py	96.98% <0.00%> (-0.18%)	⬇️
samtranslator/intrinsics/resource_refs.py	95.83% <0.00%> (-0.17%)	⬇️
... and 43 more

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[moelasmar]

@jonife .. could you please resolve the merge conflicts.