[FEATURE] Add ability for Extension to get the value of a setting from OpenSearch

[dbwiddis]

Is your feature request related to a problem?

Part of #148

In case an extension wants to use an existing OpenSearch setting as a fallback setting, it should be able to query it.

What solution would you like?

This is essentially creating Request/Response actions that do the reverse of the existing sendCustomSetting implementation. The user sends a String key and the response returns a Setting object.

Bonus points: allow querying a list of settings, for efficiency in a single transport request.

What alternatives have you considered?

Querying settings via REST API.

Do you have any additional context?

https://xkcd.com/555/

[dbwiddis]

This issue might be solved using a combination of Create Components (a settings object) and settings update consumers. Check on the work that has already been done.