Add bls serde_vistors file

eth2/utils/bls/src/serde_vistors.rs

@@ -0,0 +1,20 @@
+use hex;
+use serde::de::{self, Visitor};
+use std::fmt;
+
+pub struct HexVisitor;
+
+impl<'de> Visitor<'de> for HexVisitor {
+    type Value = Vec<u8>;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("a hex string (without 0x prefix)")
+    }
+
+    fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+    where
+        E: de::Error,
+    {
+        Ok(hex::decode(value).map_err(|e| de::Error::custom(format!("invalid hex ({:?})", e)))?)
+    }
+}