Support GCP Artifact Registry

Is your feature request related to a problem? Please describe.

Currently you cannot authenticate against the google artifact registry with the config.json file in watchtower. The reason for watchtower failing authentication is because of the expected strings in the www-authenticate header. Currently watchtower expects both a realm and a service string but the artifact registry does not send any service string when sending a HTTP GET request. example below:

This is not an issue when creating a config.json and authenticating against the artifact registry manually.

Describe the solution you'd like

Watchtower should be able to authenticate against the google artifact registry using the config.json file by not expecting a service string.

Describe alternatives you've considered

An alternative would be to build a container that have gcloud installed, this however would be a very bloated container compared to making the config.json file work

Additional context

No response