Update Container Registry fields for GCP

[kartik-579]

In the current version of dashboard, GCP type container registry is not present as an exclusive type but is used in the category "other". Also, current backend implementation only allows service-json-key for authorization. For this same implementation, username needs to be fixed which is = "_json_key". To -

1. Update GCP as a type of container registry.
2. Fix username as "_json_key" in case of GCP & block user to edit this field.
3. Display an info box to the user for the above information