Create ADR concerning Rust/Flutter interoperability

[klochowicz]

Initial exploration of flutter showed that we have a few unknowns.

Some questions:

• how to communicate with long-running tasks in Rust from Flutter, such as LDK
• where to keep state - singletons (e.g. conquer_once/lazy_static) in Rust, or keep them in Flutter and pass as parameters? (the latter is more library convention). By state I mean: things like the wallet, the db, the seed file etc.
• poll for changes or try to add some pub/sub system