Split the DKG out of FROST

[kayabaNerve]

FROST's IETF draft is uncaring towards its DKG, as-is the sign process. While the paper specifies the one implemented, there are a variety of other schemes possible which may be preferable depending on the use case.

There may also be a need to remove FROST's lagrange function for a function provided by the key-gen used. While that shouldn't be necessary for VSSS schemes, it may be beneficial with some other schemes.

Relevant to #56.

Useful for #96 which solely needs the DKG, not FROST.