Update Internal Key Implementation

[sgayangi]

Purpose

This PR updates the internal key implementation with the following changes.

• Adds the API ID as an audience claim in the internal key.
• Allows the configuration of the internal key issuer. A config is used to provide this value as shown below.

  <APIPublisher>
  ......
  <!-- Specifies the issuer of the internal key -->
  <InternalKeyIssuer>{{apim.publisher.internal_key_issuer}}</InternalKeyIssuer>
  ......
  </APIPublisher>

Fixes https://github.com/wso2/api-manager/issues/2713

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 46.96%. Comparing base (6b0a6b6) to head (79ff2ee). Report is 8 commits behind head on master.

Files	Patch %	Lines
...bon/apimgt/impl/token/InternalAPIKeyGenerator.java	50.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #12368 +/- ## ============================================ + Coverage 41.70% 46.96% +5.26% - Complexity 0 4130 +4130 ============================================ Files 1867 1867 Lines 137917 137926 +9 Branches 19936 19937 +1 ============================================ + Hits 57513 64779 +7266 + Misses 73479 65495 -7984 - Partials 6925 7652 +727 ``` | [Flag](https://app.codecov.io/gh/wso2/carbon-apimgt/pull/12368/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/12368/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | `41.74% <75.00%> (+6.34%)` | :arrow_up: | | [unit_tests](https://app.codecov.io/gh/wso2/carbon-apimgt/pull/12368/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | `20.04% <0.00%> (-0.01%)` | :arrow_down: | 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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