Add support to specify a SNS topic that belongs to a different region

[53ningen]

Issue #, if available:
#105

Description of changes:
Add support to specify a SNS topic that belongs to a different region.

I made changes in the steps written in the issue below:
#105 (comment)

Description of how you validated changes:
I verified transformed template in the steps written in DEVELOPMENT_GUIDE.

I confirmed that SAM template below could be deployed successfully.

Resources:
  WorkerFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: worker_lambda/
      Handler: app.lambda_handler
      Events:
        MessageTopic:
          Type: SNS
          Properties:
            Topic: (different region TopicArn)
            Region: (different region name)

also confirmed the SAM template below too.

Resources:
  WorkerFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: worker_lambda/
      Handler: app.lambda_handler
      Events:
        MessageTopic:
          Type: SNS
          Properties:
            Topic: (same region TopicArn)

I published a message to the SNS topic and confirmed to trigger Lambda function successfully.

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

@@             Coverage Diff             @@
##           develop     #989      +/-   ##
===========================================
+ Coverage    94.71%   94.71%   +<.01%     
===========================================
  Files           69       69              
  Lines         3160     3162       +2     
  Branches       606      607       +1     
===========================================
+ Hits          2993     2995       +2     
  Misses          85       85              
  Partials        82       82

Impacted Files	Coverage Δ
samtranslator/model/sns.py	100% <ø> (ø)	⬆️
samtranslator/model/eventsources/push.py	88.92% <100%> (+0.07%)	⬆️

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[jlhood]

Thanks so much for this contribution!