Add support for beaconAPI in lcli functions (#3252)

## Issue Addressed

NA

## Proposed Changes

Modifies `lcli skip-slots` and `lcli transition-blocks` allow them to source blocks/states from a beaconAPI and also gives them some more features to assist with benchmarking.

## Additional Info

Breaks the current `lcli skip-slots` and `lcli transition-blocks` APIs by changing some flag names. It should be simple enough to figure out the changes via `--help`.

Currently blocked on #3263.

beacon_node/beacon_chain/src/validator_pubkey_cache.rs

@@ -156,6 +156,11 @@ impl<T: BeaconChainTypes> ValidatorPubkeyCache<T> {
     pub fn len(&self) -> usize {
         self.indices.len()
     }
+
+    /// Returns `true` if there are no validators in the cache.
+    pub fn is_empty(&self) -> bool {
+        self.indices.is_empty()
+    }
 }
 
 /// Wrapper for a public key stored in the database.