common: add BOLT12 payer proof support

[vincenzopalazzo]

Summary

• 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

Testing

• make -j1 common/test/run-payer_proof
• ./common/test/run-payer_proof
• make -j1 plugins/test/run-decode_guess_type devtools/bolt12-cli plugins/offers
• ./plugins/test/run-decode_guess_type
• jq -r '.vectors[] | select(.expected.valid) | [.name, .expected.proof_bech32] | @tsv' /tmp/payer-proof-test-vectors/test_vectors.json | while IFS=$'\t' read -r name proof; do ./devtools/bolt12-cli decode "$proof" >/dev/null || { echo "$name"; exit 1; }; done
• make -j1 common/test/run-bolt12_merkle common/test/run-bolt12-format-string-test
• ./common/test/run-bolt12_merkle
• ./common/test/run-bolt12-format-string-test

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