Group deploy tokens
Description
In https://gitlab.com/gitlab-org/gitlab-ce/issues/31591 we have project deploy tokens to allow read-only access to a project. Since many times projects are organized in groups, we can consider implementing tokens at group level.
Proposal
Implement group deploy tokens, similar to https://gitlab.com/gitlab-org/gitlab-ce/issues/31591.
Group deploy tokens allow read-only access to any project in the group, even if it will be created after the token.
Tokens can then be exposed with a group variables to all the projects in the group.