Add default.pa.d folder support

Add default.pa.d folder support

The .include meta command already supports specifying a directory and
when including a directory, all files with the extension '.pa' in that
directory will be parsed in alphabetical order.

This feature can be used to add support for default.pa.d directory, so
that packages for other applications or users can just drop in a file
for configuration without changing the default.pa which is shipped.

We use the PA_DEFAULT_CONFIG_DIR for this, however, since meson quotes
this build variable, introduce an unquoted version for this purpose and
use it with .include.

Fixes: https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/issues/909
Signed-off-by: Sanchayan Maity <sanchayan@asymptotic.io>