* report_slashes plumbing in Substrate
Notably delays the SetRetired event until it provides a slash report or the set
after it becomes the set to report its slashes.
* Add dedicated AcceptedHandover event
* Add SlashReport TX to Tributary
* Create SlashReport TXs
* Handle SlashReport TXs
* Add logic to generate a SlashReport to the coordinator
* Route SlashReportSigner into the processor
* Finish routing the SlashReport signing/TX publication
* Add serai feature to processor's serai-client
Uses a full-fledged serai-abi to do so.
Removes use of UncheckedExtrinsic as a pointlessly (for us) length-prefixed
block with a more complicated signing algorithm than advantageous.
In the future, we should considering consolidating the various primitives
crates. I'm not convinced we benefit from one primitives crate per pallet.
Call and Event are both from the pallets, which are AGPL licensed. Accordingly,
they make serai-client AGPL licensed when serai-client must end up MIT
licensed. This creates a MIT-licensed variant of Calls and Events such that
they can be used by serai-client, enabling transitioning it to MIT.
Relevant to https://github.com/serai-dex/serai/issues/337.
