This patch allows configuring an "extension" string that filters
files not only by extension, but also by a subdirectory of the
standard configuration path(s).

The "extension" can now have the following format:
  "subdirectory/path/extension"
which is split on the last '/' and looks for files that end with
".extension" in "WIREPLUMBER_CONFIG_DIR/subdirectory/path/"

If no subdirectory is specified, the code behaves just like it did
before.