Add RTSP Proton CtMod

[DavidoTek]

Fix #412

Yeah, this was very straightforward to add. I made a branch here (forgot to put it under Advanced Mode): https://github.com/sonic2kk/ProtonUp-Qt/tree/rtsp-ge-proton (my fork is only for the purposes of contributing here, and as a playground, it's not a replacement for ProtonUp-Qt, purely a development fork)
I verified that it showed up, downloaded, and extracted correctly. The extracted contents have the same structure as GE-Proton, it uses tagging properly, uses the same archive format, so it was straightforward.
As a disclaimer: I am not sure if we want to go this direction though, so I won't open a PR yet. That branch is only there to serve as a proof-of-concept that it is possible, not a confirmation or endorsement :-)

[DavidoTek]

Should this be hidden behind advmode?

[sonic2kk]

I guess in this case, whether or not it goes behind Advanced Mode is more a question of "clutter" in the dropdown.

In terms of codebase "precedent", we put NorthStar Proton behind Advanced Mode, so I suppose we can do it here, too. Makes sense to me :-)

[sonic2kk]

Went ahead and pushed that change and rebased as well. Tested and it seems to work. Didn't launch any games with it but ProtonUp-Qt was able to download and extract the archive, and it looks correct and matches a standard Proton compatibility tool file structure.

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As an aside, I've mentioned a GE-Proton ctmod refactor that I've been working on in the past. This change wouldn't get in the way of that, we would be able to continue using this ctmod in the same way (since the goal would be having a GE-Proton ctmod as a base class, exactly as we do here). Just wanted to call this out :-)