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/beacon_chain/tests/sync_committee_verification.rs

@@ -28,11 +28,11 @@ lazy_static! {
 fn get_harness(validator_count: usize) -> BeaconChainHarness<EphemeralHarnessType<E>> {
     let mut spec = E::default_spec();
     spec.altair_fork_epoch = Some(Epoch::new(0));
-    let harness = BeaconChainHarness::new(
-        MainnetEthSpec,
-        Some(spec),
-        KEYPAIRS[0..validator_count].to_vec(),
-    );
+    let harness = BeaconChainHarness::builder(MainnetEthSpec)
+        .spec(spec)
+        .keypairs(KEYPAIRS[0..validator_count].to_vec())
+        .fresh_ephemeral_store()
+        .build();
 
     harness.advance_slot();