feat: allow deletion of an individual OIDC credential

[alnr]

This extends the existing DELETE /admin/identities/{id}/credentials/{type} API to accept an ?identifier=foobar query parameter for {type}==oidc like such:

DELETE /admin/identities/{id}/credentials/oidc?identifier=github%3A012345

This will delete the GitHub OIDC credential with the identifier github:012345 (012345 is the subject as returned by GitHub).

To find out which OIDC credentials exist, call GET /admin/identities/{id}?include_credential=oidc beforehand.

This will allow you to delete individual OIDC credentials for users even if they have several set up.

[alnr]

This function was moved here from identity/handler.go where it had the signature func deletCredentialWebAuthFromIdentity(identity *Identity) (*Identity, error)

[codecov]

Codecov Report

Attention: Patch coverage is 74.54545% with 14 lines in your changes missing coverage. Please review.

Project coverage is 78.19%. Comparing base (af5ea35) to head (0ee8dea).

Files	Patch %	Lines
identity/identity.go	79.16%	5 Missing and 5 partials ⚠️
identity/handler.go	42.85%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3968      +/-   ##
==========================================
- Coverage   78.21%   78.19%   -0.03%     
==========================================
  Files         363      363              
  Lines       25486    25516      +30     
==========================================
+ Hits        19935    19953      +18     
- Misses       4030     4038       +8     
- Partials     1521     1525       +4     

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[aeneasr]

This is looking great already!

What I'm most worried about is that we accidentally delete the wrong credential. While it doesn't appear to be the case from reading the code, I think it would be good to have test(s) that verify that.

To make it a bit easier to test, you can probably set up a unit test for the new identity methods?

[alnr]

I've added a test TestDeleteCredentialOIDCFromIdentity. The handler test is also quite comprehensive.