[outputs.mqtt] Allow setting last will and testament

### Use Case

Bing able to set the last will and testament means that we can tell if telegraf has disconnected ungracefully from the mqtt broker on the subscriber side.

### Expected behavior

mqtt producer plugin has options to set the last will and testament topic/payload/retain/qos which are sent to the broker on connect.

### Actual behavior

These options don't exist yet.

### Additional info

_No response_

Start by locating the MQTT producer plugin and reviewing how its existing connection options are configured and sent to the broker. Done means the plugin supports last-will topic, payload, retain, and QoS options and sends them on connect; add or update coverage where the plugin's behavior is tested.

Written by the indexing model from the issue text.