Broadcast address changes at Capella (#3919)

* Add first efforts at broadcast

* Tidy

* Move broadcast code to client

* Progress with broadcast impl

* Rename to address change

* Fix compile errors

* Use `while` loop

* Tidy

* Flip broadcast condition

* Switch to forgetting individual indices

* Always broadcast when the node starts

* Refactor into two functions

* Add testing

* Add another test

* Tidy, add more testing

* Tidy

* Add test, rename enum

* Rename enum again

* Tidy

* Break loop early

* Add V15 schema migration

* Bump schema version

* Progress with migration

* Update beacon_node/client/src/address_change_broadcast.rs

Co-authored-by: Michael Sproul <micsproul@gmail.com>

* Fix typo in function name

---------

Co-authored-by: Michael Sproul <micsproul@gmail.com>

beacon_node/beacon_chain/src/test_utils.rs

@@ -310,6 +310,21 @@ where
         self
     }
 
+    /// Initializes the BLS withdrawal keypairs for `num_keypairs` validators to
+    /// the "determistic" values, regardless of wether or not the validator has
+    /// a BLS or execution address in the genesis deposits.
+    ///
+    /// This aligns with the withdrawal commitments used in the "interop"
+    /// genesis states.
+    pub fn deterministic_withdrawal_keypairs(self, num_keypairs: usize) -> Self {
+        self.withdrawal_keypairs(
+            types::test_utils::generate_deterministic_keypairs(num_keypairs)
+                .into_iter()
+                .map(Option::Some)
+                .collect(),
+        )
+    }
+
     pub fn withdrawal_keypairs(mut self, withdrawal_keypairs: Vec<Option<Keypair>>) -> Self {
         self.withdrawal_keypairs = withdrawal_keypairs;
         self