Move init-tools.sh script into Rails repo

MRs:

1. Add to gitlab-repo repo: Move init_tools script into gitlab rails (!191251 - merged)
2. Remove from gitlab-workspaces-tools repo: Remove init_tools.sh from workspace-tools (gitlab-org/workspaces/gitlab-workspaces-tools!49 - merged)

Description

Move the contents of gitlab-workspaces-tools init-tools.sh script into Rails repo and call directly.

Benefits:

• will simplify the upcoming move of gitlab-workspaces-tools into gitlab-build-images repo
• will make it easier to maintain/update workspace variables, since the code that declares and uses them will now be in the same repo.

Acceptance criteria

• init-tools.sh should be moved to the gitlab repo and deprecated in the gitlab-workspaces-tools repo
• Take this opportunity to rename all *_EDITOR_EXTENSIONS_GALLERY_* variables to *_VSCODE_EXTENSION_MARKETPLACE_*, to follow the new standard naming convention
• Once the new script is deployed and verified on gitlab.com, open an MR to remove it from the gitlab-workspaces-tools repo and Dockerfile. This can be moved to a separate issue if desired, but it needs to get done ASAP to prevent accidental changes to the old location that are not reflected in the new one.