WIP: run against dbus-broker bench

[elmarco]

This series allows to run busd against dbus-broker benchmark, with:

DBUS_BROKER_TEST_DAEMON=path-to-busd/target/debug/busd dbus-broker/build/test/dbus/bench-connect

The most important change is the introduction of <busconfig> parsing, and then handling of a few command line options.

The test currently hangs after what looks like an initial successful connection. Further investigation needed.

/!\ This branch needs https://github.com/dbus2/zbus/pull/390

[zeenix]

@elmarco the config changes are super desirable even w/o dbus-broker tests in mind. Do you think you can clean this up soon?

[zeenix]

@elmarco I rebased this branch on latest git main and put it here. I didn't push it to your branch because I had to remove the unmerged zbus API for it to build.

[jokeyrhyme]

@elmarco howdie, I mucked about today and came up with a slightly different take on the XML configuration file work over in #146

There may or may not be something you like over there, but it's completely up to you :)