Improve Generation of the API Definition for API Products

[hisanhunais]

Purpose

• This PR improves the generation of the API definition for API products
• The following improvement were done
  • Support for $ref and nested entries of responses
  • Support for $ref and nested entries of response headers
  • Support for $ref and nested entries of examples
  • Support for property entries included in allOf, anyOf and oneOf fields of schema objects

Related Issues

• https://github.com/wso2/api-manager/issues/2470
• https://github.com/wso2/api-manager/issues/2451

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 31.08108% with 51 lines in your changes are missing coverage. Please review.

Project coverage is 47.00%. Comparing base (e4e7dff) to head (4b28a99). Report is 4 commits behind head on master.

Files	Patch %	Lines
.../carbon/apimgt/impl/definitions/OASParserUtil.java	31.08%	42 Missing and 9 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #12395 +/- ## ============================================ + Coverage 41.68% 47.00% +5.32% - Complexity 0 4158 +4158 ============================================ Files 1869 1869 Lines 138187 138229 +42 Branches 19995 20006 +11 ============================================ + Hits 57601 64976 +7375 + Misses 73605 65536 -8069 - Partials 6981 7717 +736 ``` | [Flag](https://app.codecov.io/gh/wso2/carbon-apimgt/pull/12395/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | Coverage Δ | | |---|---|---| | [integration_tests](https://app.codecov.io/gh/wso2/carbon-apimgt/pull/12395/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | `41.68% <31.08%> (+6.83%)` | :arrow_up: | | [unit_tests](https://app.codecov.io/gh/wso2/carbon-apimgt/pull/12395/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | `20.12% <14.86%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2#carryforward-flags-in-the-pull-request-comment) to find out more.

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