[feature]Support sessionVariables configuration properties

[jchrys]

Is your feature request related to a problem? Please describe.
R2DBC MySQL lacks sessionVariable configuration properties, limiting session customization compared to MySQL Connector/J.

Describe the solution you'd like
Implement sessionVariable configuration in R2DBC MySQL to allow setting session-level variables directly from the connection URL.

Additional context
mysql dev doc: https://dev.mysql.com/doc/connector-j/en/connector-j-connp-props-session.html
feature request tracked at: mirromutth/r2dbc-mysql#191