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.