[Feature/extensions] Add framework support to read extensions.yml

[saratvemulapalli]

Coming from: #1972
We now have the design of extensions.yml and would like to discover these extensions within the extensions framework.

Code Pointer:
https://github.com/opensearch-project/OpenSearch/blob/feature/extensions/server/src/main/java/org/opensearch/plugins/PluginsOrchestrator.java#L83

• Add support to read extensions
• Add unit tests

extensions.yml:

extensions:
    - name: firstExtension
      ephemeralId: 'uniqueid'
      hostName: 'myIndependentPluginHost'
      hostAddress: '127.0.0.0'
      port: '9301'
      version: '2.0.0'
   - name: secondExtension
...