Custom authenticator adapter for user authentication with external service

[Thisara-Welmilla]

Feature:

• Support for external custom authention extension #20809

With above feature, will introduce custom authenticator adapter for user authentication with external service at the application login flow.

Add following improvements to the authentication flow to incorporate authentication flow with the external authentication service:

• With above feature, we will introduce an authentication adapter which responsible for invoking external authentication service and process response.
• This adapter will support to authenticated both LOCAL and FEDERATED users and user type will be defined at runtime. The authentication flow will need to accommodate both types.

[Thisara-Welmilla]

Authenticator adapter implementation PRs:

• Add authenticator adapter for external auth services. wso2-extensions/identity-outbound-auth-adapter#1