Support for external custom authention extension

[Thisara-Welmilla]

Is your feature request related to a problem? Please describe.

Considering IS, Asgardeo, and PIC, it's a common requirement to have a capability to authenticate users with a custom authenticator that we do not provide out of the box. Currently:
For IS, the IAM developer must create an authenticator extension and add it as a binary to the product and for Asgardeo, there is no way to accomplish this.

Describe the solution you would prefer

This feature is intended to provide support for user authentication via an external custom authentication service. Such authenticator can be added to application login flow.
The product should define a contract defined for this flow that the service that extends need to implement.

Implementation Tasks:

• Support configuring the external authenticator properties with action feature Improve Action feature to utilized by the authentication extension #20931
• Custom authenticator adapter for user authentication with external service Custom authenticator adapter for user authentication with external service  #20887
• Improve current authentication architecture to accommodate the authentication extension Introduce new authenticator property: DefinedBy #21111
• Improve UI to incorporate authentication flow with the external authentication service Improve UI to incorporate authentication flow with the external authentication service #20893
• Cover unit tests for new authentication management
• Cover unit tests for new authenticator execution
• Implement and e2e test for the new authentor usages
• Documentation on external authentication service [DOC] Provide support for external custom authention extension. #21036