* 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
