pulse-server: improve fix_ flags handling

PulseAudio will convert the samplespec/channelmap to a format_info
and omits the input format/rate/channels when the fix flags are set. It
does not use the input values at all.

Do the same in pipewire-pulse, make a single format description without
the requested fields.

This also needs a session manager fix to make it deal with those missing
fields.

See #876