Add support for `sd_bus_add_object_vtable`

[arnaldo2792]

I noticed that the code to support sd_bus_add_object_vtable is commented:

rust-systemd/src/bus/mod.rs

Line 1290 in aa19581

// pub fn add_object_vtable<T: Any + 'static>(&self,

Is there any chance this was a mistake, and if not, would you be open for a contribution?. I may implement a few things using this library and I could contribute to it if the features that I need aren't present. I may want to use this other function as well:

rust-systemd/libsystemd-sys/src/bus/mod.rs

Line 251 in aa19581

pub fn sd_bus_add_object_vtable(

[codyps]

I suspect I commented it out because I was concerned I didn't quite get the lifetimes right for all the vtable interactions. I haven't used the bus module very heavily since writing it, so I have somewhat less confidence in its correctness.

I'd be happy to merge changes to it from PRs.