flutter_rust_bridge_codegen create/integrate command

As is seen in the overview and quickstart, this command automatically add Cargokit and flutter_rust_bridge-needed code into any Flutter project you specify.

Remark: Currently, the Cargokit source code has to be copied into the target repository, as is suggested officially.

:::info When Dart's native assets language feature is stabilized, we may also utilize that approach. :::