feat(rust): add flutter_rust_bridge

[bkioshn]

Description

Add flutter_rust_bridge

Description of Changes

• Add Earthfile for flutter_rust_doc including builder target and CODE_GENERATOR_WEB function
• Integrate flutter_rust_bridge to the existing Flutter project
• Add documents
• Create INSTALL_FLUTTER for Flutter, INSTALL_RUST and INSTALL_TOOLS for Rust

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[minikin]

I'm wondering if we really need to copy cargokit?

[minikin]

lgtm