`mopro init` should also work with an existing Rust project

[vivianjeng]

Problem

developers are not always creating a Rust project from scratch (e.g. current mopro init command)
they might also want to export bindings in an existing Rust project (e.g. http://github.com/privacy-ethereum/zkID or https://github.com/zkonduit/ezkl)
so that these developers can initiate the project without creating a new Rust crate

Details

example flow
user in a Rust crate
running mopro init
and it executes

1. cargo add mopro-ffi
2. create a file for targeting platform e.g.bin/ios.rs, bin/android.rs
3. add lib name and crate-type if necessary

Acceptance criteria

after mopro init, it should be able to run mopro build to generate bindings if functions have the #[uniffi::export] macro