Add BeaconChainHarness::builder (#2707)

## Issue Addressed

NA

## Proposed Changes

This PR is near-identical to https://github.com/sigp/lighthouse/pull/2652, however it is to be merged into `unstable` instead of `merge-f2f`. Please see that PR for reasoning.

I'm making this duplicate PR to merge to `unstable` in an effort to shrink the diff between `unstable` and `merge-f2f` by doing smaller, lead-up PRs.

## Additional Info

NA

beacon_node/operation_pool/src/lib.rs

@@ -632,8 +632,11 @@ mod release_tests {
         validator_count: usize,
         spec: Option<ChainSpec>,
     ) -> BeaconChainHarness<EphemeralHarnessType<E>> {
-        let harness =
-            BeaconChainHarness::new(E::default(), spec, KEYPAIRS[0..validator_count].to_vec());
+        let harness = BeaconChainHarness::builder(E::default())
+            .spec_or_default(spec)
+            .keypairs(KEYPAIRS[0..validator_count].to_vec())
+            .fresh_ephemeral_store()
+            .build();
 
         harness.advance_slot();