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.

docs/release-notes/release-notes-0.16.0.md

@@ -81,7 +81,9 @@ current gossip sync query status.
   after upgrading lnd.
 
   This feature is [opt-in](https://github.com/lightningnetwork/lnd/pull/7341)
-  via a config flag.
+  via a config flag. The status of the flag is
+  [exposed](https://github.com/lightningnetwork/lnd/pull/7402) at run-time
+  through GetInfo.
 
 * Zero-amount private invoices [now provide hop
   hints](https://github.com/lightningnetwork/lnd/pull/7082), up to `maxHopHints`