Parameter protocol updates for encoding method

[hamishwillee]

Essentially we have two parameter protocols, that pretty much differ only in that PX4 uses byte-wise encoding and ArduPilot uses C-style encoding (there are other differences, but most of these are not incompatible - more like extensions).

The suggestion by @auturgy here: mavlink/mavlink#1769 (comment) was that we simply use the protocol bits MAV_PROTOCOL_CAPABILITY_PARAM_UNION and MAV_PROTOCOL_CAPABILITY_PARAM_FLOAT to differentiate these. These can then also be used for discovery of the protocol.

I like this because it more accurately reflects the actual history - early inconsistency in the implementations, and further, it might actually have been the intent of these protocol bits originally.

Do not merge until mavlink/mavlink#1799 is finalised. If merges, will need to update the protocol bits documented.

• MAV_PROTOCOL_CAPABILITY_PARAM_ENCODE_BYTEWISE
• MAV_PROTOCOL_CAPABILITY_PARAM_ENCODE_C_CAST

[hamishwillee]

mavlink/mavlink#1799 has merged - this is the corresponding docs update.