Implement JWTCookie Refresh in App-Frontend application

[TheTechArch]

Description

The app-frontend would need to refresh the JWTCookie in intervals as long that a user is doing stuff. This to keep the user logged in.

Considerations

SBL must handle the JWT cookie and convert it to their own cookie format when navigating.
Risk for ending up with a forever living session

Acceptance criteria

• User is kept logged in as long he has a active session
• User is not kept logged in if he doest not have activity
• The refreshed JWT token is properly signed
• Expired or invalid JWT token is not refreshable

Tasks

• Implement endpoint in Platform Authentication for refreshing JWTCookie
• Implement endpoint + SI implementation for refreshing cookie in app backend
• Implement logic to call refresh endpoint at intervalls from app frontend
• Update documentation
• Manual test

[jezpoz]

Assigning myself to work on the app-frontend.
Also added team tamagotchi tag since thats the best team, because I'm on that team.

[jezpoz]

Frontend part of this task is in PR: #2609

[jezpoz]

After a review of frontend-code, Team Tamagotchi wants to make a more performant solution for this. Or at least come up with a more performant solution that is approved by our interaction designers.

The improvement issue can be worked with at a later stage.
Issue: Altinn/app-frontend-react#105

[trymen]

Tested in at21, keepAlive verified to be triggered, and user can continue the workflow. Closing this issue as completed.