Beacon state validator id filter (#1803)

## Issue Addressed

Michael's comment here: https://github.com/sigp/lighthouse/issues/1434#issuecomment-708834079
Resolves #1808

## Proposed Changes

- Add query param `id` and `status` to the `validators` endpoint
- Add string serialization and deserialization for `ValidatorStatus`
- Drop `Epoch` from `ValidatorStatus` variants

## Additional Info

Please provide any additional information. For example, future considerations
or information useful for reviewers.

common/eth2/src/lib.rs

@@ -210,12 +210,14 @@ impl BeaconNodeHttpClient {
         self.get_opt(path).await
     }
 
-    /// `GET beacon/states/{state_id}/validators`
+    /// `GET beacon/states/{state_id}/validators?id,status`
     ///
     /// Returns `Ok(None)` on a 404 error.
     pub async fn get_beacon_states_validators(
         &self,
         state_id: StateId,
+        ids: Option<&[ValidatorId]>,
+        statuses: Option<&[ValidatorStatus]>,
     ) -> Result<Option<GenericResponse<Vec<ValidatorData>>>, Error> {
         let mut path = self.eth_path()?;
 
@@ -226,6 +228,24 @@ impl BeaconNodeHttpClient {
             .push(&state_id.to_string())
             .push("validators");
 
+        if let Some(ids) = ids {
+            let id_string = ids
+                .iter()
+                .map(|i| i.to_string())
+                .collect::<Vec<_>>()
+                .join(",");
+            path.query_pairs_mut().append_pair("id", &id_string);
+        }
+
+        if let Some(statuses) = statuses {
+            let status_string = statuses
+                .iter()
+                .map(|i| i.to_string())
+                .collect::<Vec<_>>()
+                .join(",");
+            path.query_pairs_mut().append_pair("status", &status_string);
+        }
+
         self.get_opt(path).await
     }