Attribute Macro durable

#[durable]

Will drop current MLS group in memory and replace it with the one in the keystore. This simulates an application crash. Once restarted, everything has to be loaded from the keystore, memory is lost.

Requires the MlsConversation method to have a parameter exactly like backend: &MlsCryptoProvider

This helps spotting:

• when one has forgotten to call persist_group_when_changed
• if persisted fields are sufficient to pursue normally after a crash

IF you mark a method #[durable], remove its call to persist_group_when_changed and tests still pass, you either:

• have unit tests not covering the method enough
• do not require this method to be durable