Implement threshold cryptography / secret sharing for t-of-n decryption and signatures

It would be great if e.g. a team of size `n` could be configured such that a threshold of at least `t` members of the team would have to agree in order to decrypt a message which was encrypted to the team. (Perhaps using teams for this doesn't make sense, as it would also complicate adding and deleting members, but it's one approach).

E.g. then I could encrypt my secrets to some sort of public key for the team, and then a quorum of the team could decrypt it, e.g. after I died.

Supporting a threshold cryptography signature scheme would also be very handy.

The issue names no files, tests, or entry points. Start by identifying the project's existing team encryption and signing paths, then clarify the required threshold model and key-management behavior; done would mean a defined, reviewed implementation supporting t-of-n decryption and signatures.

Written by the indexing model from the issue text.