feat: add a host-based auth cache as a fallback

[ktarplee]

This PR adds a new exported method called NewRobustCache() and changes the DefaultCache to use the robust cache. The robust cache uses scoped based auth but falls back to host based auth to better handle the situations described in #650 but retain the benefits of scoped based auth (where the token for a repo might be different than another repo in the same registry).

Closes #650

[ktarplee]

This PR obviously still needs

• [x] Unit tests
• [x] Integration testing
• [x] Documentation

[shizhMSFT]

@ktarplee Would you like to update this PR to reflect the latest discussion in #650?

[ktarplee]

@ktarplee Would you like to update this PR to reflect the latest discussion in #650?

I made the name change.

[codecov[bot]]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (faaa1dd) 75.41% compared to head (25e15fc) 75.46%.

Files	Patch %	Lines
registry/remote/auth/cache.go	82.35%	4 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #651 +/- ## ========================================== + Coverage 75.41% 75.46% +0.04% ========================================== Files 59 59 Lines 5606 5640 +34 ========================================== + Hits 4228 4256 +28 - Misses 1015 1019 +4 - Partials 363 365 +2 ```

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[ktarplee]

I also added unit tests for the hostCache and exported NewSingleContextCache() function. @shizhMSFT

[shizhMSFT]

@ktarplee LoL. The DCO check failed. Could you fix that?