fix!: updates to client authentication jwt as rfc7523bis draft

[guillp]

Update client assertion authentication as defined in https://www.ietf.org/archive/id/draft-ietf-oauth-rfc7523bis-07.html :

• prefer using the AS issuer identifier as audience in the client assertions when that value is known
• warn when that value is not known
• use "client-authentication+jwt" as typ header in assertions

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2712873) to head (3a426df).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #267   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines         2066      2073    +7     
=========================================
+ Hits          2066      2073    +7     

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