[outputs.mqtt] Allow setting last will and testament

[gazpachoking]

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