feat: use dedicated ProxySQL monitor user instead of msandbox

[renecannao]

ProxySQL monitor user should be rsandbox/rsandbox (replication role) instead of msandbox/msandbox (full privileges).

The monitor user only needs USAGE + REPLICATION CLIENT for health checks. Using the full-privilege msandbox user is unnecessary and not best practice.

Also consider:

• Configure msandbox_rw as the write proxy user (HG 0)
• Configure msandbox_ro as the read proxy user (HG 1)
• For InnoDB Cluster, configure the icadmin user in ProxySQL