Adding file monitoring hooks

[corey-nm]

Note: Not integrated into server yet.

Main hook is start_file_watcher for server to call into to kick off a watcher process. Everything else is just helpers for that.

The file watcher at a high level does the following:

1. Check every X seconds if the file has been modified
2. If so, check if any endpoints were added/removed/modified
3. If endpoints have changed, send requests to server to update it live
4. Save the old config as <path>.v<version>

test plan: unit tests