feat(rest): support generateIdToken in impersonation url

[cuiy0006]

There could be different formats of impersonation URLs in the ADC credential files like:

1. base_url[:generateAccessToken]
2. base_url[:generateIdToken]
3. base_url

We already use base_url:generateAccessToken to generate access token. base_url:generateIdToken is not an alternative approach to generate access token, it is used for requesting a JWT token, then we can decode it and verify the email and aud.

No matter which of them appears in the impersonation URL in the credential file, we will ignore them, we will always use base_url[:generateAccessToken] to request access token, and use base_url[:generateIdToken] to do verification.

As mentioned by Brent, the design of impersonation URL is TBD, :generateAccessToken and :generateIdToken could be removed from the impersonation URL in the credential files. I think it makes sense to support all the formats above.

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.00%. Comparing base (8e7038e) to head (5dda374).
Report is 48 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14853   +/-   ##
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  Coverage   93.00%   93.00%           
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  Files        2351     2351           
  Lines      209479   209482    +3     
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+ Hits       194818   194825    +7     
+ Misses      14661    14657    -4     

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[scotthart]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @cuiy0006)