Implement NUMERIC support for MySQL

[abonander]

This is a lot easier than Postgres as MySQL transmits and receives NUMERIC values as decimal strings, so you can just decode the value as String and deal with parsing it later.

I think like PgNumeric, we should have a MySqlNumeric type that's just a newtype around String which implements Decode<MySql>, and then a impl TryFrom<MySqlNumeric> for BigDecimal which is gated on the bigdecimal feature.

In fact, we can probably use Cow<'_, str> in MySqlNumeric instead so we can decode a value without copying/allocation.

I can't figure out why MYSQL_TYPE_DECIMAL and MYSQL_TYPE_NEWDECIMAL both exist as types in the binary protocol but everything I can find just says to use MYSQL_TYPE_NEWDECIMAL. I'm told MariaDB may be different, though.