BIP459: Add FullAgg (DahLIAS) module

[fjahr]

This PR adds a FullAgg module implementing full aggregation of BIP 340 signatures using the interactive aggregate signature scheme (DahLIAS).

The implementation follows the BIP459.

[w0xlt]

Concept ACK

[fjahr]

The last commit here now is implementing the code sharing between the musig and the fullagg model. That is one of the first questions I have when reviewers will take a closer look here: Do we want to do it this way? And do we event want to do this at all? I think it's pretty good but also the amount of LOC saving has been a bit underwhelming at the end.

A second thing that I changed and that I would be interested in high-level feedback on: I realized that the fullagg module really doesn't need any of the Schnorr module code, it just depends on extrakeys. I fixed this in the config where I still had the schnorr module as dependency of the fullagg module. However, in practical terms they will likely always be active together and it's also a bit awkward that I still have the module named schnorrsig_fullagg which I took over from the Halfagg PR where it's schnorrsig_halfagg. The most correct way would be to remove the schnorrsig part here but I am also unsatisfied with the asymmetry this would create 🙈 If anyone has opinions on this I am happy to hear them.