pipewire: Add support for HDR capture

[flibitijibibo]

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PipeWire 1.6 and newer finally provides support for BT2020/ST2084 video capture/streaming. This should allow the existing xdg-portal/PipeWire integration to support HDR video capture from compositors exposing the new format enum values.

The catch right now is the compositor part; because 1.6 is new this is still in progress - for example, this is the draft for kwin:

https://invent.kde.org/plasma/kwin/-/merge_requests/8293

There are apparently OBS patches somewhere acting as a client-side test case, but other than that I'm not aware of any other clients implementing this yet.