Implement block producer for validator client

validator_client/src/signer.rs

@@ -0,0 +1,7 @@
+use types::Signature;
+
+/// Signs message using an internally-maintained private key.
+pub trait Signer {
+    fn sign_block_proposal(&self, message: &[u8], domain: u64) -> Option<Signature>;
+    fn sign_randao_reveal(&self, message: &[u8], domain: u64) -> Option<Signature>;
+}