Extend backends to select secret key distributions (sparse/uniform)

CKKS implementations can use sparse or uniform ternary distributions for the secret keys. This will change parameter settings and performance - for e.g. sparse keys will lower the amount of levels that bootstrapping will consume.

Right now, values are typically used for uniform ternary distributions. I don't fully know what else is changed from this (noise budgets likely?) but it may be good to have passes be able to configure this.