createinvoice: make a minimal blinded "path" in bolt12 invoice if none presented.

The "path" is just a message to ourselves.  This meets the minimal
requirement for bolt12 invoices: that there be a blinded path (at
least so we can use the path_id inside in place of "payment_secret").

We expose the method to make this path_id to a common routine: offers
will need this for generating more sophisticated paths.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/invoice_path_id.c

@@ -0,0 +1,19 @@
+#include "config.h"
+#include <bitcoin/privkey.h>
+#include <ccan/crypto/sha256/sha256.h>
+#include <common/invoice_path_id.h>
+
+u8 *invoice_path_id(const tal_t *ctx,
+		    const struct secret *base_secret,
+		    const struct sha256 *payment_hash)
+{
+	struct sha256_ctx shactx;
+	struct sha256 secret;
+
+	sha256_init(&shactx);
+	sha256_update(&shactx, base_secret, sizeof(*base_secret));
+	sha256_update(&shactx, payment_hash, sizeof(*payment_hash));
+	sha256_done(&shactx, &secret);
+
+	return (u8 *)tal_dup(ctx, struct sha256, &secret);
+}