This function will be called every time a conversation's epoch changes.
The epoch parameter is the new epoch.
Though the signature includes an error type, that error is only present because
it is required by uniffi in order to handle panics. This function should suppress
and ignore internal errors instead of propagating them, to the maximum extent possible.
OptionalasyncOpts_: { signal: AbortSignal }
An
EpochObserveris notified whenever a conversation's epoch changes.