Consume pt_config.json also for default PT options

In #42343 (closed) we started using pt_config.json to load the built-in bridge lines in this repository, rather than converting it in tor-browser-build.

However, that file contains also the default arguments for the pluggable transports, and we generate the torrc-default from them.

We could do also that here, rather than in tor-browser-build.

The biggest disadvantage is that the expert bundle consumers will also need to implement that.