Actions: MAVLink: integrate and expose relevant metadata

### Current behaviour

#1436 added basic support for MAVLink message actions, but commands are configured using just numbered fields (so it's hard to know what they should include, or whether they're even necessary), and other messages use a raw JSON, which is even less convenient.

### Expected or desired behaviour

We should automatically create entries for relevant fields, and use known metadata to show things like message descriptions, field names and units, and even field descriptions as tooltips.

@patrickelectric has been kind enough to make [this dataset](https://github.com/patrickelectric/mavlink-json/blob/master/message_definitions/v1.0/ardupilotmega.json), which auto-generates metadata JSON from the MAVLink xml files on a weekly basis. We should submodule that and import the data so we can make use of it :-)

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.

Start by reviewing the existing MAVLink message-action support from #1436 and the linked message_definitions/v1.0/ardupilotmega.json dataset. Trace how numbered fields and raw JSON are currently configured, then determine where metadata can be imported and exposed; done means relevant fields are created automatically and descriptions, names, units, and field tooltips are available.

Written by the indexing model from the issue text.