Add maxctrl command/API to start/stop CDC

Currently, stopping or starting CDC replay requires commenting out the config section and restarting MaxScale. This is disruptive and manual.

Ask: Add a maxctrl command (and corresponding REST API endpoint) to start/stop CDC replay without editing config or restarting MaxScale.

Acceptance Criteria:

• maxctrl supports a command to stop CDC replay on a running MaxScale instance without a restart.
• maxctrl supports a command to start CDC replay on a running instance without a restart.
• Equivalent REST API endpoints exist for start/stop, usable by external tooling/scripts.
• Stopping CDC does not stop the binlogrouter or affect binlog replication.
• Current CDC state (running/stopped) is queryable via maxctrl/API.