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]

Summary of Changes

Hello @GeorgeTsagk, 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 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.

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[gemini-code-assist]

Code Review

This pull request introduces support for applying both a single and double tweak to a private key, which is a useful extension. The implementation is correct. I've suggested a refactoring in maybeTweakPrivKey to simplify the logic and improve readability. Additionally, I've noticed that input.ReadSignDescriptor will return an ErrTweakOverdose if it tries to deserialize a SignDescriptor with both tweaks set. If these combined-tweak descriptors are ever serialized and deserialized, this will cause issues. You may want to consider removing that check in input.ReadSignDescriptor as part of this or a follow-up change to ensure consistency with the new behavior.