Add a Herald pre-commit field for detecting LFS usage

Summary: Depends on D18825. Ref T7789. See PHI131. Allows installs to selectively disable LFS by adding Herald rules to block commits that use LFS.

Test Plan:

• Wrote an LFS rule ("When commit uses git lfs, block commit").
• Pushed an LFS commit: rejected.
• Pushed a non-lFS commit: success.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T7789

Differential Revision: https://secure.phabricator.com/D18827