util: add helper ChannelHandshakeConfig -> ChannelHandshakeConfigUpdate

[phlip9]

Mirror the ChannelConfig -> ChannelConfigUpdate helper

[ldk-reviews-bot]

I've assigned @wpaulino as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-claude-review-bot]

I've reviewed the full diff. The From<ChannelHandshakeConfig> for ChannelHandshakeConfigUpdate impl maps all 7 fields of ChannelHandshakeConfigUpdate correctly, and the mapping is the exact inverse of the existing apply method (lines 1255-1290). The 5 ChannelHandshakeConfig fields excluded from the conversion (negotiate_scid_privacy, announce_for_forwarding, commit_upfront_shutdown_pubkey, negotiate_anchors_zero_fee_htlc_tx, negotiate_anchor_zero_fee_commitments) are correctly omitted as they have no counterpart in ChannelHandshakeConfigUpdate. Types all match.

No issues found.