feat: add Id token support for UserCredentials

[TimurSadykov]

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Fixes #492 ☕️
Fixes #468
Fixes #157

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[codecov]

Codecov Report

Merging #650 (bce601b) into master (292e81a) will decrease coverage by 0.33%.
The diff coverage is 56.00%.

❗ Current head bce601b differs from pull request most recent head 9877f93. Consider uploading reports for the commit 9877f93 to get more accurate results

@@             Coverage Diff              @@
##             master     #650      +/-   ##
============================================
- Coverage     83.59%   83.25%   -0.34%     
- Complexity      606      607       +1     
============================================
  Files            42       42              
  Lines          2712     2724      +12     
  Branches        289      291       +2     
============================================
+ Hits           2267     2268       +1     
- Misses          303      313      +10     
- Partials        142      143       +1     

Impacted Files	Coverage Δ
...ava/com/google/auth/oauth2/IdTokenCredentials.java	76.47% <0.00%> (-10.20%)	⬇️
...p/java/com/google/auth/oauth2/UserCredentials.java	76.66% <66.66%> (-3.62%)	⬇️

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