Allow modifying a Server's zbus connection builder

[DJ-Laser]

By default zbus Connections spawn a thread per connection, however this can be disabled by calling .internal_executor(false) on it's builder, and manually ticking it (in my case by spawning a future onto smol's executor).

While spawning another thread unnecessarily isn't a huge deal, being able to modify the connection builder in general could be very useful, even if gated behind the unstable feature.

Unfortunately Server::connection isn't much help, since the executor needs to be disabled before it's built, and can't be disabled after, and I don't see any good solutions to allow modifying the Server's connection that aren't breaking changes.

See:
https://docs.rs/zbus/latest/zbus/connection/struct.Connection.html#method.executor