[FEATURE] Abstract out configuration for extension in extensions.yml

[saratvemulapalli]

Is your feature request related to a problem?

Coming from opensearch-project/OpenSearch#7235 (review).
Extensions configuration has:

extensions:
  - name: hello-world
    uniqueId: opensearch-sdk-java-1
    hostAddress: '127.0.0.1'
    port: '4532'
    version: '1.0'
    opensearchVersion: '3.0.0'
    minimumCompatibleVersion: '3.0.0'

Its about time to refactor this config into abstracted types.

What solution would you like?

extensions:
  - name: hello-world
    - identity:
       uniqueId: opensearch-sdk-java-1
    - transport:
       hostAddress: '127.0.0.1'
       port: '4532'
    - compatibility:
       version: '1.0'
       opensearchVersion: '3.0.0'
       minimumCompatibleVersion: '3.0.0'

This will help enable adding new types of configuration without really changing the top level interfaces.

What alternatives have you considered?

Leave it as it is today.

cc: @dblock