feat: allow arbitrary additional claims for JwtClaims

[chingor13]

No description provided.

[codecov]

Codecov Report

Merging #331 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #331      +/-   ##
============================================
+ Coverage     79.61%   79.67%   +0.05%     
  Complexity      389      389              
============================================
  Files            27       27              
  Lines          1756     1761       +5     
  Branches        183      183              
============================================
+ Hits           1398     1403       +5     
  Misses          262      262              
  Partials         96       96

Impacted Files	Coverage Δ	Complexity Δ
...h2_http/java/com/google/auth/oauth2/JwtClaims.java	92.3% <100%> (+3.41%)	8 <5> (ø)	⬇️
...tp/java/com/google/auth/oauth2/JwtCredentials.java	84.41% <100%> (+0.2%)	13 <0> (ø)	⬇️
...om/google/auth/oauth2/ImpersonatedCredentials.java	85.48% <0%> (ø)	15% <0%> (ø)	⬇️
...uth2_http/java/com/google/auth/oauth2/IdToken.java	82.6% <0%> (ø)	10% <0%> (ø)	⬇️

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