Improve how the AAD is set

[raphaelrobert]

Currently, the AAD is set permanently at the MlsGroup level. It is persisted and applied to every outgoing message until its value is changed. The intuition is that apps would most likely like to set it per message and don't want it to be persisted, not even in RAM.

Rust doesn't offer an elegant way to handle optional arguments, we could however do the following syntax:

mls_group.with_aad(b"This is the AAD.").add_members(...)?;

Internally, with_aad() would set a temporary AAD field and consume mut self. Any function that sends out messages would honor the AAD field and clear it before returning.