Stub out rethdb receipt fetcher with `rethdb` build tag

op-service/sources/reth_db.go

+//go:build rethdb
+
 package sources
 
 import (

op-service/sources/reth_db_stub.go

+//go:build !rethdb
+
+package sources
+
+import (
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/types"
+)
+
+// FetchRethReceipts fetches the receipts for the given block hash...
+func FetchRethReceipts(dbPath string, blockHash *common.Hash) (types.Receipts, error) {
+	panic("unimplemented!")
+}