Accept ready-made JWTs in API

[lunkwill42]

A sort of MVP for JWT tokens in NAV would be to have the API simply accept valid JWT tokens as authorization:

• The API must require JWTs to be cryptographically signed by a trusted key (RS256).
• The API must require JWTs to have an expiry date claim.
• The API must require JWTs to have a not-before date claim.

SimpleJWT is a potential candidate for library to use here - BUT, SimpleJWT seems to have a dependency on the Django user model that NAV does not support. API endpoints in NAV do not perform operations on behalf of specific users, so the user model is irrelevant for NAV API atm. We might want to fork some of SimpleJWT's code to complete this MVP.

This functionality can be tested by manually constructing and signing a JWT token, submitting this to any API endpoint in NAV for authorization.

This MVP does not need to consider access claims at all. These can be defined later.