Fix: Description in AWS::Serverless::HttpApi

[hawflau]

Issue #, if available:

Description of changes:
This PR fixes how the Description property is handled for a AWS::Serverless::HttpApi resource:
When Description is set, one of the followings will happen:

• if DefinitionBody is not set, an error will be thrown
• if DefinitionBody is set and description is already set in the OpenAPI definition, an error will be thrown
• if DefinitionBody is set and no description is set in the OpenAPI definition, description will be set in the OpenAPI definition.

Before the changes, deploying a AWS::Serverless::HttpApi resource with Description set will result in an error:

Redundant fields [description="description"] provided when either Body or
BodyS3Location is not empty (Service: null; Status Code: 0; Error Code: null;
Request ID: null; Proxy: null) (Service: null; Status Code: 404; Error Code:
BadRequestException; Request ID: null; Proxy: null)  

Note that the same error does not occur for AWS::Serverless::Api, but only AWS::Serverless::HttpApi.

Description of how you validated changes:
Updated and added tests to cover all changes made

pytest tests/openapi/test_openapi.py
pytest tests/model/test_sam_resources.py

Verified transformed template deploys without error.

Checklist:

• Write/update 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.

[codecov-io]

Codecov Report

Merging #1884 (e27312e) into develop (c752104) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1884      +/-   ##
===========================================
+ Coverage    93.99%   94.01%   +0.01%     
===========================================
  Files           89       89              
  Lines         5832     5848      +16     
  Branches      1188     1192       +4     
===========================================
+ Hits          5482     5498      +16     
  Misses         162      162              
  Partials       188      188              

Impacted Files	Coverage Δ
samtranslator/model/api/http_api_generator.py	91.06% <100.00%> (+0.28%)	⬆️
samtranslator/open_api/open_api.py	92.56% <100.00%> (+0.19%)	⬆️

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