Extension point for registering the authentication strategies

[jannyHou]

Description / Steps to reproduce / Feature proposal

Follow the extension pattern example to create extension point for registering multiple authentication strategies.

Background of the story:
Now we implement a new provider in a hardcoded way to perform the authenticate action of the custom jwt strategy created by ourselves, see custom authentication provider.
This should be an extensible point that people can plugin the actions for both passport and non passport-based strategies.

Acceptance Criteria

• Create an extension point for authentication strategies
  • Decorate a strategy provider class AuthenticationStrategyProvider with @extensionPoint
  • The strategy provider class constructor should have a strategy getter decorated with @extensions
  • update the authentication action provider AuthenticateActionProvider to use AuthenticationStrategy interface and the new strategy provider class
  • Document how to implement & register an auth strategy. Can be light documentation since we already have some documentation discussing extension points and extensions.
• Use "jwt" and "basic" strategies for tests. Not shipping production ready strategies.

See Reporting Issues for more tips on writing good issues

[bajtos]

IMO, we should design the extension point for auth strategies in the following way:

• The extension point is LoopBack specific, the strategy interface is defined to match the needs of LoopBack and custom LoopBack strategies.
• To support Passport-based strategies, we should provide an adapter that will accept a Passport strategy and convert it into LB strategy (that will be part of Creating adapter for the plugged in passport strategies #2311 though).

[bajtos]

I think this story depends on #2466: Add abstraction for authentication strategy.

[jannyHou]

To support Passport-based strategies, we should provide an adapter that will accept a Passport strategy and convert it into LB strategy

Sounds good to me. There is already an adapter in @loopback/authenticate we can borrow.

[dhmlau]

@emonddr , with your PR #2763 merged, does it mean this task is done? Thanks.