assets swap deposits [1/N]: add asset HTLC helpers

[bhandras]

This PR introduces SwapKit, a helper struct for constructing and managing HTLCs used in atomic asset to bitcoin swaps. It includes support for generating success/timeout scripts, taproot control blocks, witness creation, and psbt generation with MuSig2 aggregate keys.

Pull Request Checklist

• Update release_notes.md if your PR contains major features, breaking changes or bugfixes

[gemini-code-assist]

Summary of Changes

Hello @bhandras, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request lays the groundwork for atomic asset-to-Bitcoin swaps by introducing a dedicated SwapKit and a suite of helper functions. These additions streamline the creation, management, and spending of HTLCs, providing robust support for script generation, Taproot integration, and witness construction, which are essential components for facilitating secure and efficient cross-chain asset transfers.

Highlights

• New HTLC Helper Struct: Introduced SwapKit, a central helper struct designed to manage and construct Hash Time-Locked Contracts (HTLCs) specifically for atomic asset-to-Bitcoin swaps. This struct encapsulates all necessary parameters for a swap.
• HTLC Script Generation: Added utility functions (GenSuccessPathScript, GenTimeoutPathScript) to programmatically generate the Bitcoin scripts for both the success (preimage) and timeout paths of an HTLC, with optional CHECKSEQUENCEVERIFY support for pinning attack prevention.
• Taproot Integration for HTLCs: Implemented comprehensive Taproot support for HTLCs, including functions to create Taproot leaves (GetTimeOutLeaf, GetSuccessLeaf), compute aggregate MuSig2 keys (GetAggregateKey), generate Tapscript sibling preimages (GetSiblingPreimage), and construct Bitcoin control blocks for both success and timeout spending paths (GenSuccessBtcControlBlock, GenTimeoutBtcControlBlock). An OP_TRUE Taproot leaf helper (CreateOpTrueLeaf) was also added.
• Witness and PSBT Generation: Provided methods within SwapKit to facilitate the creation of Bitcoin transaction witnesses for both the success (CreatePreimageWitness) and timeout (CreateTimeoutWitness) paths of an HTLC, integrating with an lndclient.SignerClient for signing and supporting PSBT generation.
• Asset Commitment Utilities: Included helper functions (GenTaprootAssetRootFromProof, GetPkScriptFromAsset) to derive the Taproot asset root from a proof and to generate the top-level Bitcoin script for an asset, crucial for integrating asset commitments into the Bitcoin blockchain.

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[gemini-code-assist]

Code Review

This pull request introduces the SwapKit struct to facilitate atomic asset to bitcoin swaps using HTLCs. The code includes functions for generating scripts, taproot control blocks, witness creation, and PSBT generation. The review focuses on type safety, documentation, and ensuring the correctness of signature generation.

[gemini-code-assist]

The SignDescriptor struct is being populated with WitnessScript: successScript, but the Output field is being populated with assetTxOut. It's important to ensure that the Output field is the correct output being signed, and that it corresponds to the WitnessScript being used. Otherwise, the signature will be invalid.

// Ensure that the Output field corresponds to the WitnessScript being used.
		Output:        assetTxOut,

[gemini-code-assist]

The SignDescriptor struct is being populated with WitnessScript: timeoutScript, but the Output field is being populated with assetTxOut. It's important to ensure that the Output field is the correct output being signed, and that it corresponds to the WitnessScript being used. Otherwise, the signature will be invalid.

// Ensure that the Output field corresponds to the WitnessScript being used.
		Output:        assetTxOut,

[gemini-code-assist]

Consider using the asset's Amount type directly instead of btcutil.Amount to avoid potential confusion and ensure type safety. This might require changes in other parts of the code that interact with this field.

Suggested change

	// we use btcutil.Amount here for simplicity, but the actual amount
	// is in the asset's native unit.
	Amount btcutil.Amount
	// Amount is the amount of the asset that will be swapped.
	Amount asset.Amount

[gemini-code-assist]

Consider adding a comment explaining why asset.Version(1) is being used here. Is this a default value, or is there a specific reason for using version 1?

Suggested change

	// todo(sputn1ck) assetversion
	AssetVersion: asset.Version(1),
	Amount: uint64(s.Amount),
	Interactive: true,
	AnchorOutputIndex: 1,
	ScriptKey: asset.NewScriptKey(
	tapScriptKey.PubKey,
	),
	AnchorOutputInternalKey: btcInternalKey,
	AnchorOutputTapscriptSibling: &siblingPreimage,
	})
	// AssetVersion: asset.Version(1) // TODO: Determine the correct asset version to use.