Add RTSP server

[varsill]

Currently, the membrane_rtsp package (https://github.com/membraneframework/membrane_rtsp) contains only RTSP client, capable of receiving RTSP messages.
Adding the corresponding RTSP server functionality (so that the user would be able to send RTSP messages) should be problematic and definitely it would be helpful, because we then could use such a server e.g. in our rtsp_to_hls demo.
Apart from them, I believe the package name should be converted to ex_rtsp to follow our naming conventions.

[varsill]

It has turned out that there is already an implementation of a simple RTSP server in Jellyfish. I slightly modified it and added it to the rtsp_to_hls demo: https://github.com/membraneframework/membrane_demo/blob/master/rtsp_to_hls/server.exs

[mat-hek]

@varsill should we close this then?

[varsill]

As discussed - closing since no further development is required at this moment. In the future, we might want to rewrite the RTSP server in such a manner, that each message could be responded to within a callback.

[gBillal]

Hi @varsill @mat-hek
Since I also need an RTSP server, I decided to create a fork of membrane_rtsp taking inspiration from the server example mentioned in this issue. This is the work done so far.
It's not yet finished as I'm trying to wrap my head around the RFC. If you think we can add it to membrane_rtsp I'll create a PR.

[mat-hek]

Hi, it looks good to me! Feel free to open a PR, thanks ;)