[1/2] lnwire: fix encoding customized TLV records

[yyforyongyu]

Depending on #10027, this PR fixes the wrong encoding used in the extra data field. Followup PRs will be created to fix the rest of the msgs. The msgs in this PR are not used in production yet, and for other msgs, it's likely the error wasn't caught due to there's no interoperability test yet.

The commits are structured as follows,

1. A commit that adds a unit test for the encoding/decoding - we check that the raw bytes will be decoded and encoded into the same bytes.
2. A fix in the actual encoding/decoding methods.

[gemini-code-assist]

Summary of Changes

Hello @yyforyongyu, 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 addresses a critical bug in the lnwire package where customized TLV records, particularly those in the 'extra data' fields of certain messages, were being incorrectly encoded. The changes involve a significant refactoring of the TLV handling logic, introducing a standardized approach for parsing and encoding these records. This ensures that all TLV data, whether known or unknown, is processed correctly, improving the reliability and forward compatibility of Lightning Network messages. Extensive new tests have been added to validate these fixes.

Highlights

• TLV Encoding Fix: Implemented a fix for incorrect encoding of customized TLV (Type-Length-Value) records within the 'extra data' field of various Lightning Network messages. This ensures that both known and unknown TLV fields are correctly handled during serialization and deserialization.
• Standardized TLV Parsing: Introduced a new helper function, ParseAndExtractExtraData, which centralizes the logic for separating known TLV records from arbitrary 'extra data' within a message's TLV stream. This promotes consistent and robust parsing across the lnwire package.
• TLV Type Correction: Corrected the TLV type for DALocalMusig2Pubnonce in the DynAck message from 0 to 14. This resolves a potential conflict with other TLV types and aligns with standard practices for assigning even-numbered types to optional fields.
• Enhanced Test Coverage: Added comprehensive unit tests for the encode/decode functionality of several lnwire messages, including AnnounceSignatures1/2, ChannelAnnouncement1/2, ChannelUpdate2, DynAck, DynCommit, and DynPropose. These tests verify byte-for-byte fidelity after a full encode-decode roundtrip, ensuring the correctness of the TLV handling.
• Type Refinements: Updated the TLV record types for DustLimit and ChannelReserve in dynamic commitment messages (DynPropose, DynCommit) to use tlv.BigSizeT for btcutil.Amount, providing a more appropriate and flexible encoding for these values.

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[gemini-code-assist]

Code Review

This pull request introduces a fix for encoding customized TLV records in lnwire, specifically addressing an issue where known TLV fields were incorrectly encoded into the ExtraData field of messages. The changes include adding unit tests and refactoring encoding/decoding methods to ensure correct serialization and preservation of unknown TLV fields. The code adheres to the LND Style Guide.