Mavgen WLua: Updates to displayed field values in Wireshark

[shancock884]

As a follow up to #856, this PR proposes some updates to the decoding and displaying of Mavlink fields in Wireshark:

- Convert some units into more readable values (appended to the original, so raw value is still shown), e.g.:

lon (int32_t) [degE7]: -25922213 (-2.5922213 deg)

More converted units can be added, by adding to the unit_decoder_mapping lookup table.

- Show value of bitmask fields in HEX_DEC format, and pad bitfields to a multiple of 4 to match hex, e.g.:

flags (EKF_STATUS_FLAGS): 0x033f (831)
  .... .... ...1 = EKF_ATTITUDE: True
... (etc) ...
  .0.. .... .... = EKF_UNINITIALIZED: False

- Append local date/time to μs fields if it represents a unix time, or display time in seconds, unless < 1s, e.g.:

time_unix_usec (uint64_t) [us]: 1694287362375871 (2023-09-09 20:22:42.375871 BST)
time_usec (uint32_t) [us]: 937785569 (937.785569 s)

- In header, show system id as DEC, and MAV_COMPONENT as enum, e.g.:

System id: 1
Component id: MAV_COMP_ID_AUTOPILOT1 (1)

- Decode command parameters on MISSION_ITEM messages, e.g.:

param1: Land Options (NAV_VTOL_LAND_OPTIONS): NAV_VTOL_LAND_OPTIONS_DEFAULT (0)
param2 (float): 0
param3: Approach Altitude (float) m: 0
param4: Yaw (float) deg: 0
x: Latitude (int32_t): 514775152
y: Longitude (int32_t): -25939161
z: Ground Altitude (float) m: 0

Previously this was only done for COMMAND_LONG/INT messages, but not for MISSION_ITEM(_INT) messages.

- Added 2 new automated test cases to demonstrate/check the unit decoding and MISSION_ITEM_INT fields.

[peterbarker]

Merged, thanks!