Add tags/refs/semver support for `GitRepositories`

## Description
We introduces UI support for 
https://fluxcd.io/flux/components/source/gitrepositories/#reference

We went with support for branches in v1. Now we go deeper.

## Acceptence crtieria
User can manually enter
- [ ] tags
- [ ] commit hashes
- [ ] semver

## Out of scope
- [ ] Fetch possible values from GitHub api
