refactor: Move codegen into it's own crate

Cargo.lock

Cargo.toml

Makefile

dbus_codegen/Cargo.toml

dbus_codegen/src/lib.rs

dbus_codegen/src/network_manager.rs

dbus_codegen/src/network_manager_access_point.rs

dbus_codegen/src/network_manager_connection_active.rs

dbus_codegen/src/network_manager_device.rs

dbus_codegen/src/network_manager_settings.rs

src/dbus_wrappers/active_connection.rs

src/dbus_wrappers/connection.rs

src/dbus_wrappers/device.rs

src/dbus_wrappers/manager.rs

src/dbus_wrappers/manager_settings.rs

src/main.rs