Support libpeas based plugin system

[Rafostar]

Add a libpeas based plugin system. In order to not be confused with GStreamer plugins, I will be using term enhancers as these are meant to be callable from already present functionalities in Clapper library, enhancing them (possibly using other non-C libraries). These are meant mostly for either not actual playback and/or out of GStreamer scope functionalities.

Currently implemented is CLAPPER_EXTRACTABLE, an interface for creating enhancers that extract data from different sources before letting it be played via GStreamer. Note that absolutely no actual extraction code is shipped as part of the app. For this reason loadable enhancers system was invented.

For now, this is meant to replace gtuber with something (hopefully) easier to maintain by using extractors from different libraries (that others will maintain instead of myself 😆).

---

Issues:
Needs CLAPPER_USE_PLAYBIN3=1 to play smoothly in HQ - not fixable here, we should change to playbin3 by default for next Clapper release anyway while other apps can already enforce it with env, so not a problem.

Testing:
Both local build and Flatpak package from this PR CI (checks tab) work fine. Windows is currently untested.

[Rafostar]

Currently, applications can quickly check an enhancer availability with:

gboolean
clapper_enhancer_check (
  GType iface_type,
  const gchar* scheme,
  const gchar* host,
  const gchar** name
)

AND check whether Clapper was even compiled with ability to load enhancers (requires libpeas) by using CLAPPER_WITH_ENHANCERS_LOADER or alternatively get this from pkgconfig.

Example:

gboolean supported = clapper_enhancer_check (CLAPPER_TYPE_EXTRACTABLE, "https", "example.com", NULL);

As requested in: GeopJr/Tuba#1171

ping @GeopJr

[GeopJr]

Sounds good to me, thanks! And I'm interested in maintaining a few extractors probably :)

[Rafostar]

Created a separate repo for enhancer type of plugins here. Also added there an yt-dlp enhancer implementing ClapperExtractable interface from this PR.

I will later update this PR to fix CI with missing dependencies and replace gtuber with this new stuff, so it will be easier to test.

[Rafostar]

Sounds good to me, thanks! And I'm interested in maintaining a few extractors probably :)

Thanks. Hopefully now most of the hard stuff will be handled by Clapper Enhancer yt-dlp and yt-dlp guys will maintain actual web extraction for us 😄.

Nonetheless, you are (and everyone else) always welcome to contribute (maybe even with new extractors). Now they can be written in C, Vala, Python, GJS and maybe even Rust. I will be moving the ones that are preferred to be custom written over yt-dlp from gtuber repo.

[Rafostar]

The CI now passed, Flatpak was built and... it works! 🎉
I will be updating this PR first comment description with any issues/concerns/todo's I find while testing.

[Rafostar]

OK. So what was tested overall now:

• loading/writing python plugins
• loading/writing C (compiled) plugins
• registering internal GStreamer static elements
• custom URI schemes
• passing tags, TOC and headers
• DASH streaming
• HLS streaming
• direct video URI playback

In overall, everything "here" seems to work fine. What remains is to add/restore more extractors, but these can be even gradually added later (even after next Clapper version), since its another repo and gtuber can still work as a fallback when installed for the not yet ported things.