Add a ./bin/storage shell command

Summary: Fixes T7078. Adds a ./bin/storage shell command which passes through to a MySQL shell. This is slightly more convenient than running mysql manually.

Test Plan: Ran ./bin/storage shell and got a MySQL shell.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7078

Differential Revision: https://secure.phabricator.com/D11548