How to migrate apps using loopback-component-passport

[bajtos]

This is a follow-up for #3718 and #3922.

Write content for docs/site/migration/auth/passport.md, explain how to migrate LB3 applications using loopback-component-passport to LB4.

I think the long term goal should be to move our users to @loopback/authentication-passport.

For short to medium term, while they are running both LB3 and LB4 app at the same time, we can show them how to use Passport as Express middleware, see #3947.
scoped to #5005

Acceptance criteria

Migration guide describing manual steps for the following scenarios:

• LB4-only, this should show how to migrate to our recommended setup. (PR : docs: migrate loopback-example-passport repo as lb4 example #4899)

  • add support for oauth2 redirection in loopback strategy (Support for oauth2 passport strategies, missing provisions for redirect and callback #4902)
  • create example app for login with facebook and google, similar to https://github.com/strongloop/loopback-example-passport ([Spike] Explore/research the oauth2+session based authentication strategy #2624)
  • enabling client sessions in the example app to save user profiles on login and delete session on logout

• Both LB3 and LB4 applications running in the same process, we want to use a single passport configuration to secure both LB3 and LB4 routes.
  scoped into a separate issue: Migrate apps using loopback-component-passport with LB3 Composition #5005