This commit extends ChannelEdgeInfo to support v2 (taproot) channel
announcements by adding:
- MerkleRootHash for the optional Merkle tree commitment
- ExtraSignedFields for additional signed TLV fields
- ChannelV2Fields struct to encapsulate v2-specific optional fields
- NewV2Channel constructor for creating v2 channel edges
- FundingPKScript handling for MuSig2 key aggregation with optional
taproot tweaks
