Add J1939 speed message support

[gunicsba]

I went through the discussions and this topic came up a few times:

#617
#559

I did implement something like that for the Task Controller:
AgOpenGPS-Official/AOG-TaskController@f710437

but as you can see it's more of a hack as the stack didn't offer J1939 support.

[VzdornovNA88]

Have you checked the accuracy of message delivery timing? With this approach to organizing periodic messages.

[gunicsba]

you mean if there's any delay? It wasn't noticeable maybe there's a 1-200ms difference between NMEA and the different speed interfaces but some implements have logic to average these values for sudden spikes. So these speeds aren't that time senstive.

The actual GPS position would be a different beast there I think we should aim for <10ms of delay.

[VzdornovNA88]

Yep, this is usually a problem for feedback systems(speed control, various synchronizing systems such as harvester reel,etc similar...)

I asked because it's important for us to have deterministic transmissions as well.

Of course, such modules are usually made into a closed system through a program in a single processor, but quite often project owners want a universal controller with sensors and PWM outputs without algorithms and something separate with control algorithms, accurate transmission of feedback signals over the bus in such a system becomes important.

I also met engine controllers that spammed dm1 due to non-compliance with the deadlines of the periolic messages important for the systems that were active at the moment, of course, this is less about speed messages, but generally refers to the support of j1939 transmitters in the library.