arc package: add option to specify config file

Summary: We'd like to specify a config file for deployments -- add an option for that.

Test Plan:
Built an RPM with a custom config file containing values for `root_restrict_files`, and installed it locally. Saw that the config file was installed in `/etc/watchman.json`. Created an empty
directory called `foo` and ran `watchman watch $PWD/foo`. Saw that watchman refused to watch it, claiming that none of the files in `root_restrict_files` were present.

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D902566

Task ID: 2647765

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extra : rebase_source : 10f332fa3d8cf124e43a9f4b48f80a37a8dff6a0
extra : amend_source : dbf038d98c981e0d5d08bedffc49399164f34cde