Allow specifying a registry for a given tag or branch

[lquerel]

It seems that the most commonly used syntax for tags is the git URL followed by a #.

[MadVikingGod]

Is your feature request related to a problem? Please describe.
Add a way to fetch a repository at a specific git tag. For example weaver registry resolve -r https://github.com/open-telemetry/semantic-conventions.git --tag v1.25.0

Describe the solution you'd like
By specifying a tag, weaver should not fetch main but instead the tag specified.

Describe alternatives you've considered
Currently fetching the repository manually and checking out the tag requested.

[lquerel]

In PR #257, the following syntax is now supported: source[subfolder], and an extension for tag, branch, and commit will take the following form: source@refspec[sub_folder].

I keep this issue open as the refspec notation is not yet fully supported.