osxaudiosrc: Fixes for failing init/no output on recent iOS versions

Contains a couple fixes required to get osxaudiosrc to work correctly on iOS (>=17) again. Setting up the AVAudioSession also benefits osxaudiosink because it will no longer be silenced by the OS-wide silent mode (meant to silence notifications, not multimedia).

    osxaudio: Automatically set up AVAudioSession on iOS

    A correctly configured AVAudioSession is needed on iOS to:
    - allow the application to capture microphone audio (in some cases)
    - avoid playback being silenced in silent mode

    Without this, initializing AudioUnit for capture can fail on iOS >=17 (from my testing).

    Since AVAudioSession has a lot of settings, in most cases its setup should be handled by the user/app.
    However, just to have a basic default scenario covered, let's configure the bare minimum ourselves,
    and allow anyone to disable that behaviour by setting configure-session=false on src/sink.

    osxaudio: Fix AudioOutputUnitStart() deadlock on iOS >=17

    At some point in iOS 17, this call started waiting for the first render callback (io_proc) to finish.
    In our case, that callback also takes the ringbuf object lock by calling gst_audio_ring_buffer_set_timestamp(),
    which results in a deadlock.

    osxaudiosrc: Fix render callback removal when pausing/stopping

    At least on iOS, the 'input' callback kept being called after going to PAUSED.
    Specifying the right type (like in gst_core_audio_io_proc_start()) fixes that.

    osxaudiosrc: Work around timestamps on iOS not starting from 0

    On macOS, you always get your own 'timeline' for the AudioUnit session, so timestamps start from 0.
    On iOS however, AudioUnit seems to give you a 'shared' timeline so timestamps start at a later, non-0 point in time.
    Simply offsetting seems to do the trick.

    This was causing osxaudiosrc to not output any sound on iOS.

    Regressed in 2df9283d3f2ea06af5ebd6db03a6d545cac52f19