feat: Add single funded DLC support to messaging

[matthewjablack]

What

This PR adds comprehensive support for single funded DLCs to the messaging layer, allowing for DLC contracts where only one party provides collateral.

Changes Made

Core Changes

• DlcOffer: Added singleFunded flag and related methods (markAsSingleFunded(), isSingleFunded())
• DlcAccept: Added singleFunded flag and related methods for acceptor side
• Auto-detection: Implemented logic to automatically detect single funded DLCs during deserialization
• Validation: Updated validation rules to handle single funded DLC scenarios appropriately

Single Funded DLC Characteristics

• DlcOffer: totalCollateral equals offerCollateral (offerer funds the entire contract)
• DlcAccept: acceptCollateral is typically 0 or minimal (acceptor provides little/no collateral)
• Funding validation: Different rules apply for funding amount validation

Test Coverage

• Added comprehensive test suites for both DlcOffer and DlcAccept single funded functionality
• Tests cover validation, auto-detection, serialization/deserialization, and error cases
• All existing tests continue to pass

Backward Compatibility

• Changes are fully backward compatible
• Existing DLC messages continue to work unchanged
• New functionality is opt-in via explicit marking or auto-detection

Usage Example

// Creating a single funded DLC offer
const offer = new DlcOffer();
offer.contractInfo.totalCollateral = BigInt(1e8);
offer.offerCollateral = BigInt(1e8);
offer.markAsSingleFunded();

// Auto-detection works too
const isUserFunded = offer.isSingleFunded(); // true

// Acceptor side for single funded DLC
const accept = new DlcAccept();
accept.acceptCollateral = BigInt(0);
accept.markAsSingleFunded();

This implementation enables more flexible DLC contract arrangements where one party can fund the entire contract