common: add bolt12 payer proof support

Add common-layer payer proof encode, decode, and verification support
for the `lnp` BOLT12 proof format with selective TLV disclosure via
merkle tree reconstruction.

- payer_proof_from_invoice(): creates a payer proof from a paid invoice
  by computing selective disclosure over the invoice's TLV merkle tree,
  including only required fields (invreq_payer_id, invoice_payment_hash,
  invoice_node_id, invoice_features) plus optional extra disclosures,
  then signing with the payer key
- payer_proof_decode(): decodes and fully verifies an lnp-encoded proof
  by reconstructing the merkle root from disclosed fields, leaf hashes,
  omitted TLV markers, and missing hashes, then checking the invoice
  signature, payment preimage, and payer signature
- Expose payer proof decoding through bolt12-cli and the offers plugin
  decode command with lnp1 prefix detection
- Vector-backed tests covering the published payer-proof corpus,
  including note-bearing proofs and selective disclosure of odd
  experimental TLVs

Changelog-None

Author: vincenzopalazzo

common/Makefile

@@ -81,6 +81,7 @@ COMMON_SRC_NOGEN :=				\
 	common/peer_io.c			\
 	common/penalty_base.c			\
 	common/per_peer_state.c			\
+	common/payer_proof.c			\
 	common/permute_tx.c			\
 	common/ping.c				\
 	common/plugin.c				\