Add Trusted Publishing support for self-hosted GitLab instances

[AndreKR]

Current Behavior

The latest recommendation for publishing from CI is using "trusted publishing" and crates.io does support GitLab, but there is no field to set the URL of the GitLab instance I'm using.

Expected Behavior

I'm guessing it defaults to the one run by the GitLab developers? There are lots of reasons not to use that one, so to really work, trusted publishing needs a field to set the instance URL.

Steps To Reproduce

1. Go to the page of one of your crates.
2. Go to tab "Settings".
3. Click "Add" next to "Trusted Publishing".
4. Select Publisher: GitLab.
5. Note there is no field to enter the URL of the GitLab instance.

Environment

• Browser: Chrome 152
• OS: Windows 10

Anything else?

No response

[Turbo87]

slightly related: #12261 (comment)