added support for out of tree external transport

[tridge]

this allows for the transport implementation to be outside the XRCE tree, mediated by a void* pointer

[pablogs9]

Which is the difference between this and Custom transports: https://micro-xrce-dds.docs.eprosima.com/en/latest/transport.html?highlight=custom#custom-transport ?

[richiprosima]

Build status:

• Linux
• Windows

[tridge]

my apologies, I misunderstood the custom transport, I've now switched to using custom in ArduPilot for both UDP and serial. Closing