xpay: support paying a (simple) bolt12 offer directly.

fetchinvoice is still good for detailed diagnostics and handling
recurring invoices and alternate currencies, but this covers the
"throw some sats" case well.

Changelog-Added: JSON-RPC: `xpay` can now pay a simple offer directly, rather than requiring fetchinvoice first.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/schemas/xpay.json

@@ -17,7 +17,7 @@
       "invstring": {
         "type": "string",
         "description": [
-          "bolt11 or bolt12 invoice"
+          "bolt11 or bolt12 invoice, or a bolt12 (non-recursive) offer.  If it's an offer, the invoice is fetch using `fetchinvoice` automatically."
         ]
       },
       "amount_msat": {