rpcserver: expose store final htlc resolutions flag on info call

The storage of final htlc resolutions is a safety-critical feature. By
exposing it through the GetInfo call, connected applications can
ensure that the feature is turned on and avoid relying on the assumption
that lnd was configured correctly.

lnrpc/lightning.swagger.json

@@ -4769,6 +4769,10 @@
         "require_htlc_interceptor": {
           "type": "boolean",
           "description": "Indicates whether the HTLC interceptor API is in always-on mode."
+        },
+        "store_final_htlc_resolutions": {
+          "type": "boolean",
+          "description": "Indicates whether final htlc resolutions are stored on disk."
         }
       }
     },