feat: sam support to add resource policies to api properties

[praneetap]

Issue #, if available:
#514
Description of changes:
Added support for resource policies on api definition in SAM.
Description of how you validated changes:
Transformed this template locally and deployed it on CFN successfully. I could invoke the API successfully.

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Globals:
  Api:
    Auth:
      ResourcePolicy:
        CustomStatements: [{
            "Effect": "Allow",
            "Principal": "*",
            "Action": "execute-api:Invoke",
            "Resource": "execute-api:*/*/*",
            "Condition": {
                "IpAddress": {
                    "aws:SourceIp": "1.2.3.4"
                }
            }
        }]
Resources:
  MyFunction:
    Type: AWS::Serverless::Function
    Properties:
      InlineCode: |
        exports.handler = async () => "Hello Worlds!"
      Handler: index.handler
      Runtime: nodejs8.10
      Events:
        Api:
          Type: Api
          Properties:
            Method: Put
            Path: /get

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 #1045 into develop will decrease coverage by 0.11%.
The diff coverage is 77.27%.

@@            Coverage Diff             @@
##           develop   #1045      +/-   ##
==========================================
- Coverage    94.91%   94.8%   -0.12%     
==========================================
  Files           69      69              
  Lines         3248    3274      +26     
  Branches       631     639       +8     
==========================================
+ Hits          3083    3104      +21     
- Misses          87      89       +2     
- Partials        78      81       +3

Impacted Files	Coverage Δ
samtranslator/plugins/globals/globals.py	99% <ø> (ø)	⬆️
samtranslator/model/api/api_generator.py	96.66% <100%> (+0.02%)	⬆️
samtranslator/swagger/swagger.py	95.68% <73.68%> (-1.28%)	⬇️
samtranslator/model/sam_resources.py	95.65% <0%> (+0.02%)	⬆️
samtranslator/model/apigateway.py	97.76% <0%> (+0.03%)	⬆️

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[keetonian]

Looks good, just one minor comment about a comment.

Changes needed

[jlhood]

Awesome. Thanks for iterating on this!