Expanding Taproot Miniscript and MuSig2 support

Hi! 👋

@pythcoiner put together a branch adding Taproot Miniscript and MuSig2 support, and I've been tweaking it. It's now running in [Kern](https://github.com/odudex/Kern/tree/feat/taproot-miniscript), an ESP32P4-based air-gapped signer, where Taproot Miniscript is operational, loading and validating descriptors, signing key-path and script-path.

Branch (based on `release_1.5.4`): https://github.com/odudex/libwally-core/tree/tapscript-and-musig2

We'd really love to upstream this and work together with you to mature it, we think collaborating would make it more robust and reliable for everyone, and we're happy to help in whatever way works best for you.

So the main question: Is this something you'd be open to collaborating on? If so, how would you like us to shape it (e.g. split into smaller PRs, anything else you'd need)?

Thanks for libwally-core! It's been great to build on. 🙏

Review the tapscript-and-musig2 branch based on release_1.5.4, then compare its Taproot Miniscript and MuSig2 work with the existing libwally-core entry points. The next step is to confirm with maintainers how the work should be split and what tests or interfaces are required. Done is a maintainer-agreed upstream plan; the issue does not yet define an implementation or acceptance test.

Written by the indexing model from the issue text.