fix(oauth2): do not require id_token in creds

[MeilakhSiarhei]

Only a limited number of scopes returns this field. For the rest of scopes this field is not populated, which is ok.

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This change is 

[MeilakhSiarhei]

/gcbrun

[coryan]

We should change the tests too, maybe remove this line:

google-cloud-cpp/google/cloud/internal/oauth2_authorized_user_credentials_test.cc

Line 263 in 953aefe

"id_token": "id-token-value",

[coryan]

And probably this line too:

google-cloud-cpp/google/cloud/internal/oauth2_authorized_user_credentials_test.cc

Line 226 in 953aefe

"id_token": "id-token-value",

[coryan]

/gcbrun

[MeilakhSiarhei]

I removed those 2 lines.
There is still 'id_token' at line 65. I'm not sure if it's an issue, because sometimes it returns this field.

[coryan]

I removed those 2 lines. There is still 'id_token' at line 65. I'm not sure if it's an issue, because sometimes it returns this field.

Let's go and remove that too.

[coryan]

/gcbrun

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (953aefe) 93.54% compared to head (8f76295) 93.54%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12867   +/-   ##
=======================================
  Coverage   93.54%   93.54%           
=======================================
  Files        2075     2075           
  Lines      180728   180727    -1     
=======================================
+ Hits       169067   169069    +2     
+ Misses      11661    11658    -3     

Files	Coverage Δ
...oud/internal/oauth2_authorized_user_credentials.cc	100.00% <ø> (ø)
...nternal/oauth2_authorized_user_credentials_test.cc	100.00% <ø> (ø)

... and 3 files with indirect coverage changes

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