feat: add ability to set or unset the CLIENT_FOUND_ROWS flag

[andreastedile]

Closes #3063.

Is this a breaking change?

No: it preserves current SQLx behavior.

Context

We are developing an application with "upsert" (update or insert) semantics: each request should either update an existing row or insert a new one if no matching row exists.

To implement this behavior, we use MySQL statements of the form INSERT ... ON DUPLICATE KEY UPDATE:
https://dev.mysql.com/doc/refman/8.4/en/insert-on-duplicate.html

With ON DUPLICATE KEY UPDATE, the affected-rows value per row is 1 if the row is inserted as a new row, 2 if an existing row is updated, and 0 if an existing row is set to its current values. If you specify the CLIENT_FOUND_ROWS flag to the mysql_real_connect() C API function when connecting to mysqld, the affected-rows value is 1 (not 0) if an existing row is set to its current values.

SQLx currently enables the MySQL CLIENT_FOUND_ROWS flag by default:
https://github.com/transact-rs/sqlx/blob/main/sqlx-mysql/src/connection/stream.rs#L38

which makes it impossible to distinguish whether a row was inserted as a new row or set to its current values,
because in both cases MySQL reports affected_rows = 1 when CLIENT_FOUND_ROWS is enabled.

This PR preserves current SQLx behavior, as the found_rows variable is set to true by default.