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.proto

@@ -3836,6 +3836,12 @@ message Invoice {
     Note: Output only, don't specify for creating an invoice.
     */
     map<string, AMPInvoiceState> amp_invoice_state = 28;
+
+    /*
+    Signals that the invoice should include blinded paths to hide the true
+    identity of the recipient.
+    */
+    bool blind = 29;
 }
 
 enum InvoiceHTLCState {