Add message for getting speed distribution

[emiltin]

There's currently no way to get aggregated data about speed distribution. You can only get the average for vehicles detected since last update. If you use sendOnChange (rpeviously known as updateRate=0) you get a message for each vehicles detected, and can then do the computations yourself later, but it can take a lot of bandwidth to send a message for every vehicle.

We could either add a message that provides an aggregated speed distribution for vehicles since the last update, eg:

0-5km/h:  3
5-10km/h:  76
15-20km/h:  344
25-30km/t  123
...

But should the bins be configuration..?

Or we design a more general way to fetch data about each detection since last update, that can include speed, vehicle type, direction, etc. eg:

{
  "at":"2023-09-26T12:49:01.599Z","class":"bicycle","speed":19,"direction":"southbound"},
  "at":"2023-09-26T12:49:03.234Z","class":"bicycle","speed":23,"direction":"southbound"},
  "at":"2023-09-26T12:49:24.622Z","class":"car","speed":42,"direction":"northbound"},
  "at":"2023-09-26T12:49:45.643Z","class":"bus","speed":34,"direction":"northbound"},
  "at":"2023-09-26T12:49:56.552Z","class":"bicycle","speed":23,"direction":"southbound"},
  ..
}