Support authentication with external services

Some services have their own authentication system which is not related to ESCAPE IAM. It should be possible for the user to authenticate with and access these services from within ESAP. The user should remain authenticated with ESCAPE IAM in addition to their authentication with the external service.

For example, the user may need to log in to ESO or Rubin Observatory archives to access proprietary data products. After they have logged in, they should be able to add those data products to their shopping basket, and otherwise process them as usual using ESAP.