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adds git filesystem support to filesystem source

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We want to support git in filesystem source the same way we support buckets. This will involve using (and possibly reimplementing) the filesystem provided by fsspec by default.

1. • try if the default git filesystem implementation works, if not we'll need to implement our own. the problem I see is that last modification timestamp of the file in git repo is not returned by the default fsspec impl.
2. • allow both local and remote git repositories. dlt has a good support for cloning and checking out the branches: https://github.com/dlt-hub/dlt/blob/master/dlt/common/git.py we also have support for giturlparse for remot repos
3. • initially you can add all the code (including custom fsspec filesystem) to the filesystem verified source
4. • if you need to modify library core, do it a fork from which we can install it for testing. after a review we'll merge it
5. • please reproduce all the test cases by creating a repo with the test files used by other filesystems in filesystem source