Add a command to manually sweep loop-out HTLC

[starius]

Add loopd RPC SweepHtlc and CLI command loop out sweephtlc:

$ loop out sweephtlc -h
NAME:
   loop out sweephtlc - sweep an HTLC output using the preimage success path

USAGE:
   loop out sweephtlc [options]

OPTIONS:
   --outpoint string  htlc outpoint to sweep (format: txid:vout)
   --htlcaddr string  htlc address corresponding to the outpoint
   --feerate uint     fee rate to use in sat/vbyte
   --destaddr string  optional destination address; defaults to a new wallet address
   --preimage string  optional preimage hex to override stored swap preimage
   --publish          publish the sweep transaction immediately (default: false)

Pull Request Checklist

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

[gemini-code-assist]

Summary of Changes

Hello @starius, 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 enhances the loop client by introducing a new command that allows for the manual sweeping of loop-out HTLCs. This feature empowers users with direct control over their funds, enabling them to recover HTLCs using a known preimage. Concurrently, the PR refactors and centralizes the fee calculation and clamping mechanisms for sweep transactions, ensuring that all on-chain transactions are economically sound and adhere to network relay policies.

Highlights

• New Manual HTLC Sweep Command: Introduced a new RPC SweepHtlc and a corresponding CLI command loop out sweephtlc to allow users to manually sweep loop-out HTLCs using a known preimage. This provides greater control over funds in scenarios where automatic sweeping might be delayed or fail.
• Centralized Fee Clamping Logic: Refactored fee calculation and validation logic into a new utility function utils.ClampSweepFee. This function ensures that sweep transaction fees do not exceed a certain percentage of the HTLC value and also respects the minimum relay fee, preventing uneconomical or invalid transactions.
• Comprehensive Testing: Added extensive table tests for the new sweepHtlc functionality, covering various success and error conditions, including fee clamping and publishing outcomes.
• API and Documentation Updates: Updated gRPC API definitions, gRPC gateway, Swagger documentation, and CLI man pages to reflect the new SweepHtlc RPC and command, ensuring proper integration and user guidance.

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[gemini-code-assist]

Code Review

This pull request introduces a new sweephtlc command to manually sweep a loop-out HTLC, along with the corresponding SweepHtlc RPC. The implementation is well-structured, with clear separation of concerns and good test coverage for the new functionality. A notable improvement is the refactoring of fee clamping logic into a shared utility function, which is now used across different parts of the codebase, enhancing consistency and maintainability. My review includes a suggestion to improve the performance of swap lookups, a fix for minor comment typos in the protobuf definition, and a recommendation to enhance test coverage for the new fee utility.