[Feature/extensions] Add getSettings support for AD

[dbwiddis]

Companion PR: opensearch-project/opensearch-sdk-java#147

NOTE: Merging this PR will break main on SDK project until the companion PR is merged. Merge the SDK PR shortly after merging this one.

Description

Registers custom Settings from an extension as dynamic settings in the Settings Module.

The pattern is similar to registering REST actions, with the complication that a Setting<T> needs to be passed over transport, and due to type erasure, <T> cannot be determined at runtime. A WriteableSetting class determines this type by examining the default value and implements it for all types expected in the AD extension.

There will be a need to handle additional types in the future, such as List, and to track parsers and validators. This is tracked in #148

Issues Resolved

Fixes #79

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[dbwiddis]

This is ready for review. Notes:

• Gradle Check is failing on BWC probably associated with [AUTO] [main] Added bwc version 2.3.1. #4513
• There is still more work to be done on SDK side to complete the linked issue, but that should not hold up merging this
• It may be preferable to merge this earlier to unblock @joshpalis work on [Feature/extensions] Adding support for Extension Create Components [environment settings and add setting update consumers] #4517 which needs some classes in this code and touches many of the same methods.

CC: @saratvemulapalli @owaiskazi19

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[owaiskazi19]

Did initial pass. @dbwiddis and @saratvemulapalli do you think we should commit WriteableSetting to main?

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[dbwiddis]

Did initial pass. @dbwiddis and @saratvemulapalli do you think we should commit WriteableSetting to main?

I put in the minimum to handle the settings for AD, but there will be more work required in that class for the more general cases. See #148.

[owaiskazi19]

Great to see tests for all the types

[owaiskazi19]

Small Nit. Rest LGTM!

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[saratvemulapalli]

Thanks @dbwiddis , I did a pass most of the changes look good to me.