feat: support lfs

[thxCode]

With the development of AI, more and more Git repositories use LFS to store model weights. The ADD command already supports pulling from Git URLs, https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/reference.md#adding-files-from-a-url. This PR intends to support the ADD command to pull content managed by Git LFS.

[crazy-max]

Thanks! Left some comments, I don't have much knowledge with LFS but happy to see some support here.

Would it be possible to add tests?

[crazy-max]

I think we should check if lfs is supported (git >= 1.8.3.1): https://github.com/git-lfs/git-lfs/wiki/Installation#requirements

[crazy-max]

Should we check if there are files being tracked by LFS first?

git lfs ls-files

Or maybe look for .gitattributes with filter=lfs like:

*.png filter=lfs diff=lfs merge=lfs -text

[crazy-max]

Or maybe git lfs status?

[crazy-max]

https://github.com/moby/buildkit/actions/runs/10241968157/job/28396722664?pr=5212#step:8:5671

=== FAIL: frontend/dockerfile TestIntegration/TestGitProvenanceAttestation/worker=containerd/frontend=builtin (0.86s)
    dockerfile_provenance_test.go:305: 
        	Error Trace:	/src/frontend/dockerfile/dockerfile_provenance_test.go:305
        	            				/src/util/testutil/integration/run.go:97
        	            				/src/util/testutil/integration/run.go:211
        	Error:      	Received unexpected error:
        	            	failed to read dockerfile: failed to install lfs for repo at /tmp/bktest_containerd3672960127/root/io.containerd.snapshotter.v1.overlayfs/snapshots/2/fs: git stderr:
        	            	git: 'lfs' is not a git command. See 'git --help'.

[thxCode]

https://github.com/moby/buildkit/actions/runs/10241968157/job/28396722664?pr=5212#step:8:5671

I will try to address your comments, thanks.