Add support for SSKR (Sharded Secret Key Reconstruction)

[AndreasGassmann]

Currently, we use a custom implementation of SSS (Shamir's Secret Sharing) because there was no standard at the time.

We cannot use SLIP-39, because it does not allow to "split up" an existing BIP-39 seed.

The new bc-sskr standard covers our use case.

SSKR has some more advanced features like "groups" and "group thresholds". But for simplicity, we should only support one level for the shard generation.

In any case, we will continue supporting the reconstruction of shares that have been created using our current implementation.