Add RTSP video stream support

[patrickelectric]

Describe the project you are working on

Create a game like Mario kart home edition

Describe the problem or limitation you are having in your project

There is no real time video source support

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Adding support to RTSP video stream

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

No idea, but it would be nice to have "rtsp://IP:port/video0" as a valid source for the video stream

If this enhancement will not be used often, can it be worked around with a few lines of script?

Nops

Is there a reason why this should be core and not an add-on in the asset library?

Not sure

[Calinou]

Moved to discussions, as there is no concrete technical implementation provided.

Related to #4710 (which is about making Godot able to act as a RTSP server, not client).

[patrickelectric]

There is this awesome plugin by well known @Williangalvani
https://github.com/Williangalvani/udp-rtsp-gdextension