Expose functions to update stake in C API

[iquerejeta]

Currently the stake related to an StmInitializer or StmSigner cannot be updated using the C API. Expose such functions.

[abailly-iohk]

This would be needed by Mithril nodes in order to cope with changes in stake distribution over time. Could alternatively be handled by recreating the data structures from different parameters?

[iquerejeta]

The problem of recreating the data structures from the different parameters, is that then the keys are re-generated. What we wanted is to be able to update the parameters of the StmInitializer or StmSigner without having to re-generate the keys.

I'm thinking that it might be of interest to consider a function that takes a seed to generate the StmInitializer or StmSigner. In that way the keys are generated with respect to the seed, but the parameters can be changed.

[abailly-iohk]

In general, it seems to me it makes sense to be able to pass the keys, do you see any problem with that in general? If we have #44 then it would not be a problem right? Also, I could imagine node operators storing the keys elsewhere, perhaps in a HSM or KMS?

[iquerejeta]

I was initially inclined to not make it possible to unlink the key pair from the StmSigner instance. The idea is to minimise the amount of functions we expose to the C API. If we allow too many actions from C, then we are loosing many benefits of rust. But also allowing all these changes increases the chances of shooting yourself in the feet.

However, I'm starting to think that this might be a practical functionality. I'll open a different issue to track that.