Add fork of checkout action with enhanced capabilities

[zklaus]

This is a fork of the actions/checkout action.
The main addition is the ability to apply filters to submodule handling and the ability to do single branch checkouts also for non-shallow checkouts.
For this, it adds the single-branch and submodules-filter option as described in the Usage section of the updated README.md.

The purpose is to use these extensions to cut down on checkout time in the multitude of ci jobs that need to check out the pytorch repository as discussed in pytorch/pytorch#169184.

The updated README.md file also includes instructions for future updates to newer versions of the upstream action.

Some timings are taken from pytorch/pytorch#167737:

Workflow	Time on viable/strict	Time here	Time Saving
Lint/lintrunner-noclang	2m42s	55s	1m47s
linux-jammy-py3.14-clang12/build (via _linux-build)	2m30s	1m29s	61s
linux-jammy-cuda12.8-cudnn9-py3.10-clang12 / build (via _linux-build)	2m31s	1m33s	59s

In other words, we can save ~1m per checkout where shallow checkouts are not feasible, which includes build and test jobs on trunk and pull.

With a total of 276 jobs on trunk at the moment and ~90 jobs on a pull request, assuming ~70 PRs per day, the cumulative savings with savings of 1 minute per checkout could be between 75 and 150 hours.

[vercel]

@zklaus is attempting to deploy a commit to the Meta Open Source Team on Vercel.

A member of the Team first needs to authorize it.

[atalman]

Could you please include stats, what is the time saving when checking out single instance pytorch and small calculation of time saving per PR in average.
Also would be nice to include the details of this project in Wiki here: https://github.com/pytorch/test-infra/wiki
How to onboard and how to maintain the action in runbook style approach.

[atalman]

lgtm