DKG Removals (#467)

* Update ValidatorSets with a remove_participant call

* Add DkgRemoval, a sign machine for producing the relevant MuSig signatures

* Don't use position-dependent u8s yet Public when removing validators from the DKG

* Add DkgRemovalPreprocess, DkgRemovalShares

Implementation is via a new publish_tributary_tx lambda.

This is code is a copy-pasted mess which will need to be cleaned up.

* Only allow non-removed validators to vote for removals

Otherwise, it's risked that the remaining validators fall below 67% of the
original set.

* Correct publish_serai_tx, which was prior publish_set_keys in practice

substrate/client/src/serai/validator_sets.rs

@@ -112,4 +112,20 @@ impl<'a> SeraiValidatorSets<'a> {
       validator_sets::Call::<Runtime>::set_keys { network, key_pair, signature },
     ))
   }
+
+  pub fn remove_participant(
+    network: NetworkId,
+    to_remove: Public,
+    signers: Vec<Public>,
+    signature: Signature,
+  ) -> Vec<u8> {
+    Serai::unsigned(&serai_runtime::RuntimeCall::ValidatorSets(
+      validator_sets::Call::<Runtime>::remove_participant {
+        network,
+        to_remove,
+        signers,
+        signature,
+      },
+    ))
+  }
 }