Draft: RFC: Autoprobing data sources (frameservers)

This is a proposal in form of pseudocode to allow drivers to have autoprobers that autoprobe not only xrt_devices but also data sources. In the current state, a data source is equivalent to a xrt_fs.

Reasons

There are drivers that:

1. expose (or could expose) data sources but no xrt_device at all (vf, v4l2)
2. expose both data sources and devices that might be dependant on those data sources (realsense, euroc).

The idea of this proposal is to allow drivers to expose to the prober xrt_source_infos as they already expose xrt_devices. A xrt_source_info has the necessary information to create and start a data source (a xrt_fs) at a later time through what was before called p_open_video_device (now called p_open_source).

This proposal comes after stumbling multiple times with how it is necessary to hardcode functionality into p_list_video_devices to allow "video devices" (data sources) to be created and start. This change should make it easier for drivers to tell Monado they have sources of data to expose, and even to let other parts of the runtime consume those sources (even other drivers).

What's in the MR

The changes in this MR are mostly in pseudocode form and they should how all of this would work. vf, v4l2 and realsense drivers were adapted to show how drivers could expose their own sources for autoprobing.

In any case, this is just a proposal feel free to point out any deep flaws I might not have considered that would make this unfeasible.