Is your feature request related to a problem?

There is a sample plugin for custom settings. We should make that plugin an extension in the sample extensions project.

This will require adding capability to send lists, communicate parsers and validators, group settings, get settings to an extension from OpenSearch, and other more advanced Setting manipulation than is needed for the AD plugin, but this should be one of the first things worked on during the "preview" period.

What solution would you like?

Demonstrate custom setting registration similarly to https://github.com/opensearch-project/OpenSearch/tree/main/plugins/examples/custom-settings

Requirements:

• Create writeable implementations for parser that implement both Function<String, T> and Writeable interfaces [FEATURE] Create writeable implementations for parser  #349
• Demonstrate custom validators in HelloWorld Extension [FEATURE] Demonstrate custom validators in HelloWorld Extension #350
• Create writeable implementations for default value that implement both Function<Settings, String> and Writeable interfaces [FEATURE] Create writeable implementations for default value  #351
• Add the ability to pass a list: [FEATURE] Implement custom list settings for extensions #155
• Implement Group Settings: [FEATURE] Implement group settings for extensions #156
• Retrieve Settings from OpenSearch: [FEATURE] Add ability for Extension to get the value of a setting from OpenSearch #157 (NOTE: This may be implemented via CreateComponent (settings) and settings update consumers.)

What alternatives have you considered?

Doing it earlier: not needed for AD plugin.
Doing it later: key functionality needed for the near future.

Do you have any additional context?

Probably also dependent on Integration Test capability.