[DOC] Provide support for external custom authention extension.

[Thisara-Welmilla]

Feature:

• Support for external custom authention extension #20809

Following must be documented.

• Update the action management APIs with new action retrieval by action id API.
• Update the authenticators and IDP related APIs docs with new authenticator property type: DefinedBy

[ashanthamara]

Add API docs for action retrieval by action id API.

• Update the action management APIs with new action retrieval by action id API. docs-is#4737