[openapi-yaml] Prerequisities for Swagger 2 template.

Created by: MichalFoksa

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

I am looking for a way to convert from Openapi 3 to Swagger 2 specification. I think openapi-yaml generator with custom template is a good starting point.

In order to start, I needed to modify current generator to:

1. Group operations by resourcePath (they are grouped by tag currently) so that operations with same path are rendered one after the other: Example:

      /pet:
        post:
          .....
        put:
          .....
      /pet/findByStatus:
        get:
        .....

And not (it might happen when grouped by a tag):

      /pet:
        post:
        .....
      /pet/findByStatus:
        get:
        .....
      /pet: 
        put:
        .....

2. Created a Mustache lambda OnChangeLambda to return fragment only when current fragment is different than previous fragment. It will be used to render endpoint path only for a first operation in group.

3. Registered lambdas like lowercase, onchage (above one), uppercase, etc.

Later I am hoping to submit Swagger 2 template a new library.