Add API endpoint to count statuses of all validators (#3756)

## Issue Addressed

#3724

## Proposed Changes

Adds an endpoint to quickly count the number of occurances of each status in the validator set.

## Usage

```bash
curl -X GET "http://localhost:5052/lighthouse/ui/validator_count" -H "accept: application/json" | jq
```

```json
{
  "data": {
    "active_ongoing":479508,
    "active_exiting":0,
    "active_slashed":0,
    "pending_initialized":28,
    "pending_queued":0,
    "withdrawal_possible":933,
    "withdrawal_done":0,
    "exited_unslashed":0,
    "exited_slashed":3
  }
}
```

beacon_node/http_api/src/ui.rs

@@ -0,0 +1,71 @@
+use beacon_chain::{BeaconChain, BeaconChainError, BeaconChainTypes};
+use eth2::types::ValidatorStatus;
+use serde::{Deserialize, Serialize};
+use std::sync::Arc;
+use warp_utils::reject::beacon_chain_error;
+
+#[derive(Debug, Default, PartialEq, Clone, Serialize, Deserialize)]
+pub struct ValidatorCountResponse {
+    pub active_ongoing: u64,
+    pub active_exiting: u64,
+    pub active_slashed: u64,
+    pub pending_initialized: u64,
+    pub pending_queued: u64,
+    pub withdrawal_possible: u64,
+    pub withdrawal_done: u64,
+    pub exited_unslashed: u64,
+    pub exited_slashed: u64,
+}
+
+pub fn get_validator_count<T: BeaconChainTypes>(
+    chain: Arc<BeaconChain<T>>,
+) -> Result<ValidatorCountResponse, warp::Rejection> {
+    let spec = &chain.spec;
+    let mut active_ongoing = 0;
+    let mut active_exiting = 0;
+    let mut active_slashed = 0;
+    let mut pending_initialized = 0;
+    let mut pending_queued = 0;
+    let mut withdrawal_possible = 0;
+    let mut withdrawal_done = 0;
+    let mut exited_unslashed = 0;
+    let mut exited_slashed = 0;
+
+    chain
+        .with_head(|head| {
+            let state = &head.beacon_state;
+            let epoch = state.current_epoch();
+            for validator in state.validators() {
+                let status =
+                    ValidatorStatus::from_validator(validator, epoch, spec.far_future_epoch);
+
+                match status {
+                    ValidatorStatus::ActiveOngoing => active_ongoing += 1,
+                    ValidatorStatus::ActiveExiting => active_exiting += 1,
+                    ValidatorStatus::ActiveSlashed => active_slashed += 1,
+                    ValidatorStatus::PendingInitialized => pending_initialized += 1,
+                    ValidatorStatus::PendingQueued => pending_queued += 1,
+                    ValidatorStatus::WithdrawalPossible => withdrawal_possible += 1,
+                    ValidatorStatus::WithdrawalDone => withdrawal_done += 1,
+                    ValidatorStatus::ExitedUnslashed => exited_unslashed += 1,
+                    ValidatorStatus::ExitedSlashed => exited_slashed += 1,
+                    // Since we are not invoking `superset`, all other variants will be 0.
+                    _ => (),
+                }
+            }
+            Ok::<(), BeaconChainError>(())
+        })
+        .map_err(beacon_chain_error)?;
+
+    Ok(ValidatorCountResponse {
+        active_ongoing,
+        active_exiting,
+        active_slashed,
+        pending_initialized,
+        pending_queued,
+        withdrawal_possible,
+        withdrawal_done,
+        exited_unslashed,
+        exited_slashed,
+    })
+}