registration: Support config setings that are relative paths

registration: Support config setings that are relative paths

Add support for configuration settings that are relative paths to the
extension directory. The most common use-case for this is where the
extension ships a generic default file, but sysadmins or packagers may
want to override it. Setting `"path": true` on the config definition
will make the value be interpreted as a relative path.

And add tests!

Bug: T100956
Change-Id: Ia87ced25b35be7a314ee2937e0ccc63f9acc5bb5