Draft: Add Composer registry smoke test

Adds a Composer registry smoke test.

In it's current form, this is probably not ready. There are several issues with it:

• Composer is heavily reliant on the git protocol. Package versions from Composer actually correspond to git tags - so it is necessary for the composer.json to reside in the root of the smoke test project.
• Package names for Composer need to be unique at the instance level. The names cannot be generated dynamically because the change needs to exist in the git repository before you create the Composer package.
• Since CI job tokens cannot create git tags, this smoke test relies on the master branch. Unfortunately that means that once you run the smoke test once, you can no longer create another Composer package without deleting the previously generated package.

Once gitlab-org/gitlab#250633 is implemented in 14.5 - that will add support for virtual registries which will solve the majority of the issues here.