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/attestation_verification.rs

@@ -14,7 +14,6 @@ use int_to_bytes::int_to_bytes32;
 use state_processing::{
     per_block_processing::errors::AttestationValidationError, per_slot_processing,
 };
-use store::config::StoreConfig;
 use tree_hash::TreeHash;
 use types::{
     test_utils::generate_deterministic_keypair, AggregateSignature, Attestation, BeaconStateError,
@@ -41,12 +40,11 @@ fn get_harness(validator_count: usize) -> BeaconChainHarness<EphemeralHarnessTyp
     // not all.
     spec.target_aggregators_per_committee = 4;
 
-    let harness = BeaconChainHarness::new_with_store_config(
-        MainnetEthSpec,
-        Some(spec),
-        KEYPAIRS[0..validator_count].to_vec(),
-        StoreConfig::default(),
-    );
+    let harness = BeaconChainHarness::builder(MainnetEthSpec)
+        .spec(spec)
+        .keypairs(KEYPAIRS[0..validator_count].to_vec())
+        .fresh_ephemeral_store()
+        .build();
 
     harness.advance_slot();