feat(mpc): mpc threshold signatures

[faddat]

For now, please don't review this PR

You can think of it as me tracking my work :)

• cargo update
• clippy
• latest stable rust

What this PR does 📖

So far, it does very little, other than let the Uplink community know I would like to use it for a mpc threshold signature project that combines "the chat for the multisig" and the mpc tools for making any blockchain account into a threshold multisig w/ key rotation.

It seems to fold neatly into the "extensions" concept.

I will push to have a POC asap. Thanks for your work this is a really neat project.

Which issue(s) this PR fixes 🔨

• Resolve #

Special notes for reviewers 🗒️

Additional comments 🎤