feat(inputs): Add Mavlink input plugin

[chrisdalke]

Use Case

It would be useful to have a Mavlink input plugin which connects to a MavLink-compatible flight controller such as as ArduPilot or PX4. and translates all incoming messages into metrics.

This would allow Telegraf to be used to ingest live flight metrics from unmanned systems (drones, planes, boats, etc.) on a companion computer.

Telegraf is already often used on flight computers (eg a Raspberry Pi) to collect system metrics for drones and it would be valuable to extend this to also provide a convenient way to record flight telemetry.

Expected behavior

The Mavlink plugin should:

• Take in a connection string and other relevant settings
• Start an sync connection to Mavlink
• Convert any received Mavlink messages into metrics
• Gracefully handle disconnection/reconnection without requiring a telegraf restart

Actual behavior

N/A

Additional info

I am creating a PR to add this plugin.