Merge configuration with defaults on provider create

[jhrozek]

Currently we require that the whole provider configuration is passed on creating a provider which is awkward because then the user needs to pass e.g.:

{
  "github-app": {},
  "auto_registration": {
    "entities": {
      "repository": {
        "enabled": true
      }
    }
  }
}

it would be better if the default provider configuration (github-app in this case) was merged in by the provider.