POC: AnsattPorten as AuthenticationSource in Gitea

[nkylstad]

Description

Set up AnsattPorten as AutheticationSource in Gitea via Site Administration.
Use staging.altinn.studio as test environment

Points to investigate:

• Can we log in using AnsattPorten?
• Can we access the necessary tokens returned by AnsattPorten?
• Can we access data about available organizations for a user?
• Can we authenticate with maskinporten self-service portal using credentials we receive?
• Can we support logging in with AnsattPorten this way for existing users?

Dependencies

• We need to set up an AnsattPorten test-client that we can use