Remove NBF claim requirement from jwt token

[nicburgt]

Hi!

This PR makes the nbf claim in the JWT token optional, since the OAUTH2 specification also defines it as optional. If the nbf claim is present in a JWT token it will be validated by the jsonwebtoken library through the validation object.

If the nbf claim is not present, validation for the nbf claim is skipped as it is an optional claim.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.99%. Comparing base (cf03f03) to head (2ee492c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #252   +/-   ##
=======================================
  Coverage   80.99%   80.99%           
=======================================
  Files          42       42           
  Lines        6092     6094    +2     
=======================================
+ Hits         4934     4936    +2     
  Misses       1158     1158           

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