Add command to start MySQL on linux

Summary:
Allow for the creation of a local MySQL instance by just running a unprivileged command.

Will do something similar for mac+launchd soon.

Depends on D3874

Test Plan:
nix run .#mysql-up to bring up the server
journalctl -f --user-unit comm-mysql to view mysql logs
nix run .#mysql-down to bring the server down

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3902