Motivation:
We decided to repurpose the `MirrorCredential` to manage all repository credentials, not just those specific to mirroring. To reflect this role, we should remove `Mirror` prefix from the class name.

Caveat: This commit must be deployed to central dogma replicas after applying the changed from line#1030.

Modifications:
- Renamed `MirroringCredential` to `Credential` and moved its package.
- Removed `hostnamePatterns` property in `Credential`.

Result:
- The renamed `Credential` class can now be used for managing various types of repository credentials, beyond just mirroring.