lnrpc: add wallet_synced field to GetInfoResponse

In this commit, we add a new `wallet_synced` boolean field to the
GetInfoResponse message. This field exposes the wallet's internal sync
state with the backing chain source, providing visibility into whether
the wallet has caught up to the current chain tip.

This is distinct from the existing `synced_to_chain` field, which
represents a composite sync state that also considers the router and
blockbeat dispatcher. The new field allows callers to distinguish
between wallet sync delays and other subsystem sync states.

lnrpc/lightning.swagger.json

@@ -5556,6 +5556,10 @@
         "store_final_htlc_resolutions": {
           "type": "boolean",
           "description": "Indicates whether final htlc resolutions are stored on disk."
+        },
+        "wallet_synced": {
+          "type": "boolean",
+          "description": "Whether the wallet is fully synced to the best chain. This indicates the\nwallet's internal sync state with the backing chain source."
         }
       }
     },