Add bridges configuration section

~~Obviously there is no implementation of this yet, so the CI will fail.

But I thought it would be a good idea to show what I think will end up here.~~

This provides the ability to enable/disable bridges, and configure bridge lines. But there is not yet anything for pluggable transport configuration, which I guess we will do separately like in C Tor.

Nothing actually looks at this config yet since we're still working on the actual implementation, obviously.