feat(auth): Add ProgrammaticSourcedCredentials

[harkamaljot]

As part of allowing the user to provide their own subject token provider, this PR makes following changes:

1. Make SubjectTokenProvider trait public.
2. A new builder ProgrammaticSourcedCredentials is created to utilize this trait and a corresponding test case has been added.

In future PR, more fields will be added to the builder for ProgrammaticSourcedCredentials

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.48%. Comparing base (c4f3439) to head (cab006f).
Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
src/auth/src/credentials/external_account.rs	90.32%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2489      +/-   ##
==========================================
- Coverage   95.51%   95.48%   -0.03%     
==========================================
  Files          79       80       +1     
  Lines        3185     3214      +29     
==========================================
+ Hits         3042     3069      +27     
- Misses        143      145       +2     

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[sai-sunder-s]

Since they are already providing part of the config through code, we should provide them a way to build the full credential through code.

[harkamaljot]

Yes, I plan to do that in the next PR, as mentioned in the PR description.

[sai-sunder-s]

Lets add all the required builder methods as part of this PR. Lets do the method for loading from a config later. We can add that from config one if we get customer feedback that it is needed.

[sai-sunder-s]

Instead of making optional here, it would be better to create another struct just for programmatic and do not include credential_source in it.

[sai-sunder-s]

If you have a ProgrammaticExternalAccountFile struct which doesn't have credential_source, then you can combine that with the custom subject token provider and build the ExternalAccountConfig.

I believe that will make the code nicer, you do not have to pass source separately to make_credentials and pull it out of ExternalAccountConfig.

[harkamaljot]

Sorry I don't follow this comment do you mean to say do the following

1. create a new CredentialSource i.e. CredentialSource::Programmatic
2. Now we do not remove CredentialSource from the ExternalAccountConfig but rather, when ProgrammaticExternalAccount.into() is called it automatically converts to ExternalAccountConfig with source set as CredentialSource::Programmatic
3. ProgrammaticBuilder::Build method now calls the make_credentials rather than calling the make_credentials_from_source method?