[uniffi] Expose a UniFFI-compatible API

[Martin Geisler]

Description of feature:

I would like to use mls-rs from languages like Swift and Kotlin — and UniFFI seems like a great tool for this. It auto-generates high-level Rust bindings in a number of languages.

Use case:

Swift and Kotlin bindings will make it easy to create iOS and Android apps which use mls-rs.

Implementation discussion (Optional)

I'm planning on creating a small crate which makes the necessary adaptations of the mls-rs API to make it compatible with UniFFI.