Add devfile validator

Closes - Reserve gl namespace in devfile to avoid potent... (#409036 - closed)

MRS:

• Rails - Add devfile validator (!119056 - merged)
• GA4K - gitlab-org/cluster-integration/gitlab-agent!805 (16c57e5a) (updated directly on remote_dev branch in that repository. Will co-ordinate with Hunar to merge it to master)
• GitLab Workspaces Proxy - Change labels to lookup on service (gitlab-org/remote-development/gitlab-workspaces-proxy!21 - merged)
• Docs - gitlab-org/remote-development/gitlab-remote-development-docs@e43b7c4c

This MR does the following

• Move the devfile_processor.rb file in the RemoteDevelopment::Workspaces::Create module.
• Add bugfix to only allow injector of editors in 1 container even if there are multiple attributes to inject the editor. This makes it more reliable.
• Update devfiles to use latest version 2.2.0
• Add a devfile validator to reserve the namespace gl in the devfile spec to be able to reliably add features(e.g. injecting editors, injecting project cloner, etc.)