Remove Build command requirement for service principal

[lbussell]

Sorry this PR is so large!

Related: https://github.com/dotnet/docker-tools/issues/1264

Notes:

• Addresses #1265 - DockerService never needs auth, ManifestService uses auth when we have it, therefore I've separated the two classes out so that DockerService is never concerned about auth.
• Create a wrapper around ManifestService (now InnerManifestService) so extension methods can be mocked for testing (DockerService previously served this function).
• Add a ManifestServiceFactory to handle passing auth to the ManifestService in one place.
• Add RegistryCredentialsProvider which uses logic similar to RegistryContentClientFactory to dynamically get username/password for docker login.

TODO:

• [x] RegistryContentClientFactory Tests are failing
• [ ] Re-add implementation for image layers caching in ImageDigestCache - maybe rename to ManifestServiceCache
• [ ] Test in the public

[dotnet-issue-labeler[bot]]

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[lbussell]

The force push contains no changes, I just did a rebase instead of a merge by accident.

I ran a sanity check of the build command and it seems OK - [internal link] - another one without Windows so it can run publishing commands

We should merge to get the actual ImageBuilder image built and continue validating. We can address https://github.com/dotnet/docker-tools/issues/1296 at a later date.