[PROPOSAL] Refactor `MLCommonsClientAccessor` to make it open for extension

[br3no]

What/Why

What are you proposing?

Cf. https://github.com/opensearch-project/neural-search/pull/710#discussion_r1635185058

What users have asked for this feature?

This is a requirement from @martin-gaievski.

What problems are you trying to solve?

The change in https://github.com/opensearch-project/neural-search/pull/710 surfaced this problem. If every addition of a new parameter leads to a new method needing to be written, this makes changes very expensive.

What is the developer experience going to be?

Improved.

Are there any security considerations?

None.

Are there any breaking changes to the API

No. This is an internal class.

What is the user experience going to be?

No change.

Are there breaking changes to the User Experience?

No.

Why should it be built? Any reason not to?

What will it take to execute?

Create a POJO class to encapsulate the parameters, making it easier to extend functionality.

Any remaining open questions?

No.