Ability to do sparse checkout for git artifact

# Summary

I would like an option for git artifacts to do sparse checkouts so only certain files can be pulled down. This would allow the creation of a step that looks at a certain file in the repo and then launches another step based on the result of that without having to clone the entire repo.

A sparse checkout can be done like this:
```
git clone -b $branch_name -n $git_url /tmp/repo
cd /tmp/repo
git sparse-checkout set $file # example: 'build.yml'
git checkout
```

This can be accomplished using an init container that performs the above steps into an emptyDir, but it would be nice to have a built-in option.

# Use Cases

I am creating a build pipeline triggered by Gitea and would like to configure a generic webhook at the owner level instead of each individual repo. The workflow would contain a step to look at the file build.yml at the root of the repo and then have a step to launch a specific workflow based on the type of build it should run (Maven, NPM, Docker, etc). This would allow a decision to be made without fully cloning the repo twice. This in turn would make it easier to make a single webhook instead of one per build type.

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**Message from the maintainers**:

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Start by tracing how git artifacts are configured and fetched, then compare that flow with the issue's init-container and emptyDir workaround. Define how a built-in sparse-checkout option should accept a path such as build.yml and verify that only the requested files are fetched while existing artifact behavior remains unchanged.

Written by the indexing model from the issue text.