feat: implement DLC splicing functionality

[matthewjablack]

What

Implements DLC splicing functionality that enables using existing DLC funding outputs as inputs to new DLCs

Key Features

• DlcInputInfo struct: Represents existing DLC funding outputs for reuse
• Enhanced PartyParams: Added dlc_inputs_info field for DLC inputs
• CreateSplicedDlcTransactions: Main function for creating DLCs with spliced inputs
• Signature handling: Complete signing workflow for multisig DLC inputs
• Fee management: Proper weight calculation for P2WSH multisig inputs (~220 bytes)

Splicing Scenarios Supported

1. Splice-in: Add additional funds to create larger DLC
2. Splice-out: Remove funds to create smaller DLC
3. DLC transition: Change oracle, terms, or participants while reusing funds

Implementation Details

• Converts DLC inputs to regular transaction inputs with proper serial ID ordering
• Maintains compatibility with existing DLC transaction creation logic
• Supports multiple DLC inputs per party
• Handles signature ordering based on public key lexicographic order

Testing

• 4 comprehensive test cases covering all splicing scenarios
• Signature creation and verification tests
• Edge case handling for fee calculations