Add support for Google Artifact Registry

[paololazzari]

Fixes #1934

[paololazzari]

This pull requests allows for the GCP's Google Metadata Server to be used as a token_url:

config:
  BinderHub: 
    DockerRegistry:
      url: "https://us-central1-docker.pkg.dev"
      token_url: http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/token
      username: "oauth2accesstoken"

[paololazzari]

@consideRatio could you take a look please? this change is the only way I can get binderhub to work with artifact registry

[paololazzari]

@rgaiacs could you take a look please?

[consideRatio]

@paololazzari Sorry I am too low on time to help at the moment =/

[rgaiacs]

could you take a look please?

I never used Google Compute Engine. I cannot help. Sorry.

[paololazzari]

Is there anyone who can help? it's a very small change.

[manics]

Sorry for the delay in reviewing. Based on your comments in #1934 this seems reasonable. However rather than adding a conditional here I think it'd be cleaner to create a subclass, especially if we need to add more conditionals for other registries in future.

e.g.

class GoogleArtifactRegistry(DockerRegistry):
    # Please add some comments about where the URL comes from etc, e.g. link to the Google Cloud documentation

    @default("token_url")
    def _default_token_url(self):
        ....

    async def _get_token(self, client, token_url, service, scope):
        ....

This also makes it easier to use- you can just specify the registry class instead of having to set the token_url since that'll be embedded in the class.

[paololazzari]

@manics Thanks for the feedback, I've made those changes.

[manics]

Thanks, and once again sorry for the delay.