interfaces: add support for snapped pipewire

[sergio-costas]

A snapped pipewire requires to be able to create the pulse folder inside the global XDG_RUNTIME_DIR folder (not the snap own folder). Also, it requires to be able to create both the pipewire-[0-9]-manager and its lock file.

This patch adds that capability to the new pipewire interface, and also modifies the audio-playback interface; thus, a snap offering those slots will be able to create the corresponding global sockets, and a snap using them will be able to access the global ones.

This patch is required to snap pipewire, which is a must for Core Desktop: https://github.com/sergio-costas/pipewire-snap

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