feat: Implement mysql connection attributes

[tripplet]

Implements the support for connection attributes for mysql.

Up for discussion

• The default could be changed that the default is to not send any connection attributes.
• The library default attribute keys/values
• The ergonomics/names on the MySqlConnectOptions struct (.attribute(..), no_default_attributes(..), clear_custom_attributes

WIP, needs more tests but wanted to get some initial feedback