Proposed BIP for MuSig2 PSBT Fields

The email discusses the concept of anti-exfil and how it relates to libwally's protocol for ecdsa sigs. The protocol is described in more detail at two links: https://wally.readthedocs.io/en/release_0.8.9/anti_exfil_protocol/ and https://github.com/BlockstreamResearch/secp256k1-zkp/blob/master/include/secp256k1_ecdsa_s2c.h. It is suggested that in order to implement anti-exfil, a PSBT_IN_S2C_DATA_COMMITMENT item should be provided before filling in MUSIG2_PUB_NONCE, followed by PSBT_IN_S2C_DATA and PSBT_IN_NONCE_TWEAK. These items need to have specific relationships in order to ensure security.The email also mentions the desire for musig capable signers to handle s2c/anti-exfil and tweaks/adaptor-sigs immediately instead of waiting for the next release. For signers who don't require these features, the only difference would be adding the tweak to the musig nonces before hashing/signing, which is straightforward. Therefore, if this feature were specified, it would be an easy win. However, it is emphasized that this should not be considered a blocker.The email concludes with a link to another idea for formatting tables: https://github.com/ajtowns/bips/blob/d8a90cff616d6e5839748a1b2a50d32947f30850/bip-musig2-psbt.mediawiki.