Rename optimistic_sync.go to execution_engine.go (#10537)

* Rename optimistic_sync.go to execution_engine.go

* Update BUILD.bazel

beacon-chain/blockchain/BUILD.bazel

@@ -5,13 +5,13 @@ go_library(
     srcs = [
         "chain_info.go",
         "error.go",
+        "execution_engine.go",
         "head.go",
         "head_sync_committee_info.go",
         "init_sync_process_block.go",
         "log.go",
         "metrics.go",
         "new_slot.go",
-        "optimistic_sync.go",
         "options.go",
         "pow_block.go",
         "process_attestation.go",
@@ -103,13 +103,13 @@ go_test(
         "blockchain_test.go",
         "chain_info_test.go",
         "checktags_test.go",
+        "execution_engine_test.go",
         "head_sync_committee_info_test.go",
         "head_test.go",
         "init_test.go",
         "log_test.go",
         "metrics_test.go",
         "mock_test.go",
-        "optimistic_sync_test.go",
         "pow_block_test.go",
         "process_attestation_test.go",
         "process_block_test.go",