Make DKG secret packages serializable

[conradoplg]

Allow serializing secret packages.

Discussed in #711

^{Originally posted by 0x330a July 27, 2024}
It would be great to also allow serialization of the round 1 / 2 secret packages so that the key generation didn't have to be all at once in the same process for more asynchronous key generation. Currently trying to figure out how to do this as we can get the coefficients in round one but not reconstruct a SecretPackage outside of the library. Maybe I am missing something but I would be interested if anyone else has encountered or overcome this