feat(registry): Add generic registry for source, not target

[rattboi]

This allows auth to a generic source registry, such as dockerhub.

This partially solves #50

[codecov]

Codecov Report

Attention: Patch coverage is 17.77778% with 37 lines in your changes are missing coverage. Please review.

Project coverage is 28.60%. Comparing base (1bfe93a) to head (0f613c4).
Report is 7 commits behind head on main.

Files	Patch %	Lines
pkg/registry/generic.go	28.00%	18 Missing ⚠️
pkg/config/config.go	0.00%	11 Missing ⚠️
cmd/root.go	0.00%	3 Missing ⚠️
pkg/types/types.go	0.00%	3 Missing ⚠️
pkg/registry/client.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #737      +/-   ##
==========================================
- Coverage   29.24%   28.60%   -0.65%     
==========================================
  Files          11       12       +1     
  Lines         824      867      +43     
==========================================
+ Hits          241      248       +7     
- Misses        558      594      +36     
  Partials       25       25              

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[rattboi]

Somehow I missed that this PR exists already:

#612

Any way we could get that one across the line? Currently running into dockerhub rate-limiting and really need to be able to auth k8s-image-swapper against docker.io

[estahn]

Somehow I missed that this PR exists already:

Which one do you think makes more sense to look at?

I'm trying to find some time this week.

[rattboi]

Somehow I missed that this PR exists already:

Which one do you think makes more sense to look at?

I'm trying to find some time this week.

I'd say definitely the other one. It does everything this one does, and more, and has better tests. It seems you've reviewed the other PR previously and there was just a papercut left.

If the OG author isn't receptive or has disappeared, and that's the only issue left, ping me and I'll pull their branch and make the changes so we can get this across the finish line.