Introduce more granular interface into credentials package

[Wwwsylvia]

Currently we pass the credentials.Store interface as the input of the registry.Credential() function, but this is not the best implementation. Ideally we should pass a more granular interface that provides the Get() method.

Since changing the function signature is considered breaking, we will leave it to v3.

oras-go/registry/remote/credentials/registry.go

Lines 71 to 80 in a15da41

	// Credential returns a Credential() function that can be used by auth.Client.
	func Credential(store Store) auth.CredentialFunc {
	return func(ctx context.Context, hostport string) (auth.Credential, error) {
	hostport = ServerAddressFromHostname(hostport)
	if hostport == "" {
	return auth.EmptyCredential, nil
	}
	return store.Get(ctx, hostport)
	}
	}