Validate swagger path item objects

[aahung]

Issue #, if available:

Description of changes:

Add validation of path item objects when creating the SwaggerEditor object.

Possible concerns:

What if it breaks some existing template despite they are invalid, customers are still using them?

I just did a test with CFN, turns out even we don't do validation ourselves, CFN/Api Gateway will fail.

Tested templates:

# Failed with rollback: Invalid OpenAPI input
AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31

Resources:
  ApiWithInvalidPath:
    Type: AWS::Serverless::Api
    Properties:
      StageName: Prod
      DefinitionBody:
        swagger: 2.0
        paths:
          /foo: ""

# Failed with rollback: Invalid OpenAPI input
AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31

Resources:
  ApiWithInvalidPath:
    Type: AWS::Serverless::Api
    Properties:
      StageName: Prod
      DefinitionBody:
        swagger: 2.0
        paths:
          /foo: "some string"

# Failed with rollback: Invalid OpenAPI input
AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31

Resources:
  ApiWithInvalidPath:
    Type: AWS::Serverless::Api
    Properties:
      StageName: Prod
      DefinitionBody:
        swagger: 2.0
        paths:
          /foo: 123

# Failed during creating changeset
AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31

Resources:
  ApiWithInvalidPath:
    Type: AWS::Serverless::Api
    Properties:
      StageName: Prod
      DefinitionBody:
        swagger: 2.0
        paths:
          /foo:

# Failed during creating changeset
AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31

Resources:
  ApiWithInvalidPath:
    Type: AWS::Serverless::Api
    Properties:
      StageName: Prod
      DefinitionBody:
        swagger: 2.0
        paths:
          /foo: null

Description of how you validated changes:

Checklist:

• Add/update tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• 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.

[torresxb1]

nice!