common: add bolt12 payer proof support

Add common-layer payer proof encode, decode, and verification support
for the `lnp` BOLT12 proof format. Expose payer proof decoding through
`decode`, `bolt12-cli`, and type guessing in the offers plugin.

Add vector-backed payer proof tests covering the published payer-proof
corpus, including note-bearing proofs and selective disclosure of an odd
experimental TLV.

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				\