refactor(controller): cleanup of the credentials packages

[krancour]

After implementing a half dozen credential helpers, the commonalities between them have become more clear, so I have finally given them a common interface now and things get quite tidied up this way.

These changes are entirely structural and should not change any functionality.

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.35945% with 73 lines in your changes missing coverage. Please review.

Project coverage is 46.37%. Comparing base (6f3a220) to head (859334b). Report is 4 commits behind head on main.

Files	Patch %	Lines
internal/credentials/kubernetes/database.go	74.22%	18 Missing and 7 partials :warning:
...nternal/credentials/kubernetes/ecr/pod_identity.go	25.00%	18 Missing :warning:
...als/kubernetes/gar/workload_identity_federation.go	53.84%	12 Missing :warning:
internal/credentials/kubernetes/ecr/access_key.go	70.00%	6 Missing :warning:
internal/credentials/kubernetes/github/app.go	78.94%	4 Missing :warning:
internal/credentials/kubernetes/basic/basic.go	72.72%	2 Missing and 1 partial :warning:
.../credentials/kubernetes/gar/service_account_key.go	83.33%	3 Missing :warning:
cmd/controlplane/controller.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2161 +/- ## ========================================== - Coverage 46.57% 46.37% -0.20% ========================================== Files 240 242 +2 Lines 16731 16744 +13 ========================================== - Hits 7792 7765 -27 - Misses 8565 8606 +41 + Partials 374 373 -1 ```

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