fix: make usageplan properties referable

[ShreyaGangishetty]

Issue #, if available:
#1432

Description of changes:
made usageplan resources a referable property to api

Description of how you validated changes:

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 #1458 into develop will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           develop    #1458   +/-   ##
========================================
  Coverage    94.36%   94.36%           
========================================
  Files           78       78           
  Lines         4685     4685           
  Branches       937      937           
========================================
  Hits          4421     4421           
  Misses         121      121           
  Partials       143      143

Impacted Files	Coverage Δ
samtranslator/model/sam_resources.py	94.06% <100%> (ø)	⬆️

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[praneetap]

Please add docs on referable properties

[ShreyaGangishetty]

Please add docs on referable properties

I will submit another PR for docs on referable properties that will also have other referable properties documented for AWS::Serverless::Function, AWS::Serverless::Api and AWS::Serverless::HttpApi