multi: add blinded paths to invoices

Expose the ability to add blinded paths to an invoice. Also expose
various configuration values.

We also let the lncfg.Invoices struct satisfy the Validator interface so
that we can verify all its config values in one place.

lnrpc/lightning.swagger.json

@@ -5490,6 +5490,10 @@
           },
           "description": "Maps a 32-byte hex-encoded set ID to the sub-invoice AMP state for the\ngiven set ID. This field is always populated for AMP invoices, and can be\nused along side LookupInvoice to obtain the HTLC information related to a\ngiven sub-invoice.\nNote: Output only, don't specify for creating an invoice.",
           "title": "[EXPERIMENTAL]:"
+        },
+        "blind": {
+          "type": "boolean",
+          "description": "Signals that the invoice should include blinded paths to hide the true\nidentity of the recipient."
         }
       }
     },