Data plane verification (#4639)

* Add signature to blob

* Change Signable trait to support returning references to signable data

* Add signing to broadcast

* Verify signatures in window_service

* Add testing for signatures to erasure

* Add RPC for getting current slot, consume RPC call in test_repairman_catchup for more deterministic results

core/src/cluster.rs

@@ -1,6 +1,8 @@
+use solana_client::thin_client::ThinClient;
 use solana_sdk::pubkey::Pubkey;
 
 pub trait Cluster {
     fn get_node_pubkeys(&self) -> Vec<Pubkey>;
+    fn get_validator_client(&self, pubkey: &Pubkey) -> Option<ThinClient>;
     fn restart_node(&mut self, pubkey: Pubkey);
 }