R1:7198bd7db784

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.