feat: allow for configuring the docker hub registry endpoint via API

[tuunit]

Comprehensive Summary of your change

This PR adds the possibility to change the registry endpoint of DockerHub via API. This is useful for Harbor instances that run in airgapped environments and have to go through a DMZ or use a reverse proxy to access DockerHub.

Why not just use a normal Docker Registry type instead of the DockerHub adapter?

Because of the logic for library images which only applies to the Registry Type DockerHub:

harbor/src/server/middleware/repoproxy/proxy.go

Lines 128 to 144 in 8bf710a

	func defaultLibrary(ctx context.Context, registryID int64, a lib.ArtifactInfo) (bool, string, error) {
	if registryID <= 0 {
	return false, "", nil
	}
	reg, err := registry.Ctl.Get(ctx, registryID)
	if err != nil {
	return false, "", err
	}
	if reg.Type != model.RegistryTypeDockerHub {
	return false, "", err
	}
	name := strings.TrimPrefix(a.Repository, a.ProjectName+"/")
	if strings.Contains(name, "/") {
	return false, "", nil
	}
	return true, name, nil
	}

By allowing to change the registry endpoint for DockerHub, those kinds of environments can still benefit from the library substitution logic.

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[tuunit]

/label release-note/enhancement

[tuunit]

@Vad1mo any feedback on the approach? Do you want me to write an apitest?

[Vad1mo]

Makes sense to me..

It might be the stupid questions. but why is the URL hardcoded and not changeable in the first place?
Why not add the convenience to the user to preset it and allow the user to enter what he wants.

[tuunit]

I have no idea why it is hardcoded 😅 If you want me to change the UI as well to allow changing it. I can do it as part of this PR or another. Just let me know what you guys would prefer on how to continue