lnwallet: support combined tweak to private key

[GeorgeTsagk]

Description

Previously we'd define either a single or a double tweak for the sign descriptor. We introduce the option to apply both consecutively (double tweak first, single tweak second) if both tweak parameters are set. For callers who define only one of the two parameters we maintain the old behavior.

[gemini-code-assist[bot]]

Summary of Changes

Hello @GeorgeTsagk, I'm Gemini Code Assist[^1]! 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 lnwallet's private key handling by allowing the sequential application of both double and single tweaks to a private key. This provides greater flexibility in key derivation scenarios where both types of tweaks are required, while explicitly defining the order of application to ensure predictable outcomes. The change maintains compatibility with existing implementations that utilize only one type of tweak.

Highlights

• Combined Private Key Tweak: Introduced the ability to apply both a double tweak and a single tweak consecutively to a private key within the maybeTweakPrivKey function.
• Defined Tweak Order: When both tweaks are present, the double tweak is applied first, followed by the single tweak, ensuring a consistent derivation process.
• Backward Compatibility: The existing behavior for applying only a single tweak or only a double tweak remains unchanged, preserving compatibility for current callers.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with :thumbsup: and :thumbsdown: on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for Github and other Google products, sign up here.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

[^1]: Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution.

[lightninglabs-deploy]

@roasbeef: review reminder @georgetsagk, remember to re-request review from reviewers when ready