feat: Returning an issuer claim on request errors

[TimurSadykov]

Adding additional data to debug issues, where token request intermittently returns invalid_grant errors.

This does not fix the #654 completely, but a step forward.

Fixes #654 ☕️

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[codecov]

Codecov Report

Merging #656 (26a1fcb) into master (292e81a) will decrease coverage by 0.10%.
The diff coverage is 58.33%.

❗ Current head 26a1fcb differs from pull request most recent head 16d34c3. Consider uploading reports for the commit 16d34c3 to get more accurate results

@@             Coverage Diff              @@
##             master     #656      +/-   ##
============================================
- Coverage     83.59%   83.48%   -0.11%     
- Complexity      606      607       +1     
============================================
  Files            42       42              
  Lines          2712     2719       +7     
  Branches        289      289              
============================================
+ Hits           2267     2270       +3     
- Misses          303      307       +4     
  Partials        142      142              

Impacted Files	Coverage Δ	Complexity Δ
.../google/auth/oauth2/ServiceAccountCredentials.java	82.55% <58.33%> (-0.96%)	68.00 <1.00> (+1.00)	⬇️

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[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[googlebot]

CLAs look good, thanks!

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