feat: add sync relay guard fanout gating and env config

.env.example

@@ -13,6 +13,7 @@ POOL_SIZE=20
 # PARRHESIA_POLICIES_AUTH_REQUIRED_FOR_WRITES=false
 # PARRHESIA_POLICIES_AUTH_REQUIRED_FOR_READS=false
 # PARRHESIA_POLICIES_MIN_POW_DIFFICULTY=0
+# PARRHESIA_SYNC_RELAY_GUARD=false
 # PARRHESIA_FEATURES_VERIFY_EVENT_SIGNATURES=true
 # PARRHESIA_METRICS_ENABLED_ON_MAIN_ENDPOINT=true
 # PARRHESIA_METRICS_PRIVATE_NETWORKS_ONLY=true