[Feature]: Example gubbins repository

### User Story

As a developer of an extension I want to have a clear view of how to develop and maintain my extension.

### Feature Description

The `extensions/gubbins/` directory should be copied to a dedicated repo each time a release is made.

### Definition of Done

- [ ] New gubbins repository in DIRAGGrid
- [ ] Releases of DiracX trigger an update of the gubbins repo
- [ ] Gubbins has it's own GitHub actions CI, like how extensions should
- [ ] Gubbins is mirrored to CERN GitLab so the example GitLab CI config gets tested

### Alternatives Considered

Maintaining gubbins within the diracx repo was the original idea however it quickly became apparent that this wasn't practical as:

* we can't really test CI workflows from a subdirectory
* `hatch-vcs` gets metadata from a parent rather than `.git/`
* pixi locking doesn't work properly (due to the hatch-vcs issue)

We also end up putting gubbins in the top level `pixi.toml` file regardless as we want to be able to easily install and test with PRs against diracx in a different way to what is needed in extensions.

### Related Issues

_No response_

### Additional Context

_No response_