src/daemon: move the optional drop-ins to conf.avail
These drop-ins are not meant to be enabled by default, so let's move them to fontconfig style *.conf.avail/ subfolders from which they can be copied or symlinked to a location that will get merged into the corresponding configuration file.
I have checked and it appears that it's possible to create relative symbolic links e.g. ln -s ../pipewire.conf.avail/10-rates.conf
and PipeWire will load drop-ins linked in such fashion.