Add multi_mavsdk_server.sh util

[antonio-sc66]

Bash script that eases spawning multiple mavsdk_server processes. Useful to perform multi vehicle testing, both in SITL and real executions.

Instances are spawned following the next pattern:

./mavsdk_server -p 50040 udp://:14540
./mavsdk_server -p 50041 udp://:14541
./mavsdk_server -p 50042 udp://:14542

Mavsdk-Server executable name can easily be selected by editing a constant, same with the spawned process name.

It Kills the already running mavsdk_server instances when it's launched and then clears the spawned processes at exit

I use it in conjunction with scripts like sitl_multiple_run.sh

Execution example:
User has the mavsdk_server_manylinux2010-x64 binary in the parent folder and executes the script:

./multi_mavsdk_server.sh -n 3
Killing already running mavsdk-server instances
Spawning: 3 instances
Instance:  0 udp://: 14540 -p 50050  PID: 114123
Instance:  1 udp://: 14541 -p 50051  PID: 114124
Instance:  2 udp://: 14542 -p 50052  PID: 114125
[09:12:27|Info ] MAVSDK version: v1.4.9 (mavsdk_impl.cpp:20)
[09:12:27|Info ] MAVSDK version: v1.4.9 (mavsdk_impl.cpp:20)
[09:12:27|Info ] Waiting to discover system on udp://:14540... (connection_initiator.h:20)
[09:12:27|Info ] Waiting to discover system on udp://:14542... (connection_initiator.h:20)
[09:12:27|Info ] MAVSDK version: v1.4.9 (mavsdk_impl.cpp:20)
[09:12:27|Info ] Waiting to discover system on udp://:14541... (connection_initiator.h:20)

[julianoes]

Nice! Thanks for the contribution @antonio-sc66.