When "utf8mb4" is available, use it as the default client charset when invoking standalone "mysql" commands

Summary:
Fixes T13390. We have some old code which doesn't dynamically select between "utf8mb4" and "utf8". This can lead to dumping utf8mb4 data over a utf8 connection in bin/storage dump, which possibly corrupts some emoji/whales.

Instead, prefer "utf8mb4" if it's available.

Test Plan: Ran bin/storage dump and bin/storage shell, saw sub-commands select utf8mb4 as the client charset.

Maniphest Tasks: T13390

Differential Revision: https://secure.phabricator.com/D20742