feat: custom jwt claims

[lfleischmann]

Description

Add the ability do define custom claims in the session token.

Implementation

• Add a new configuration option session.jwt_template that allows defining session.jwt_template.claims.
  Values can be arbitrarily nested and can use Go text templates.
• The custom claims are also added to sessions/validate endpoint responses.
• Modified the jwt create command to pretty print the payload alongside the token.

Test

• Some unit tests for the new code are included.
• For manual testing:
  • configure claims in the config file then use the jwt create command to see if the claims are added to the token
  • use the generated token with the sessions/validate endpoints to see if the claims are also part of the response