GAR integration: services layer

🔥 Problem

The client class should not be exposed directly but accessed by services.

🚒 Solution

Implement or evolve the following services:

• get details of a single repository.
• get list of docker images.
  • Additional validations
    • The page_size parameter should be a number.
    • The order_by parameter has a finite set of possible values.
• get details of a single docker image.
  • Additional validation: the name parameter is mandatory.

Those services should have a common validations:

• check for the GAR feature flag.
• check for the google cloud support saas only feature.
• user permissions should be checked.
• client class errors should be properly handled (with a ServiceResponse.error).