Update obs-plugintemplate build stuff, add GH workflow

[mauvealerts]

This incorporates the pile of recent improvements to obs-plugintemplate build process. There are a couple areas to maybe tweak, but I'd like feedback before I bother. Most importantly: does this seem desirable/welcome?

Some notes:

• CMakeLists.txt is the only modified file, everything else is newly-added
• There is now a GitHub workflow to build the plugin. Currently, it's configured to run when any branch is pushed
• The file produced by the workflow requires a build from OBS head. There are ABI incompatible changes, yet again
• Currently builds for both x86 and x64
• MacOS and Linux builds are disabled. I left most of the infrastructure in-place for clearer merging with obs-plugintemplate
• cmake-format checks are disabled. WIL doesn't comply, and I haven't looked into excluding it
• The new builds are complicated, but getting more automation seems desirable
• You might need to update cmake and/or msvc
• Name and version currently appear in both buildspec and cmake

Re: needing OBS head: I tried pinning to the latest release and ran into a complaint about missing libcurl (via deps/file-updater). If desired, I think this can be fixed by pinning obs-deps closer to the 27.2 branch point. More generally, I currently don't understand the relationship between obs-studio and obs-deps. This stuff is new and (IMO) a bit underdocumented.

Re: duplication of name/version: I'm not sure it's possible to have just one, but I'm willing to fiddle with it some more.

[mauvealerts]

I did look into making this work with the 27.2.x branch of OBS, but it's not straightforward (at least with my knowledge). Considering this plugin will probably be obsolete for OBS 28, I'm closing this PR.