Do we need a constructor with both NamedGroup and SigAlgParamSpec? NamedGroup is for EC and SigAlgParamSpec is for RSASSA-PSS, they shouldn't co-exist in the same scheme.

There are currently 5 constructors in SignatureScheme class, I find the ordering of arguments somewhat confusing. The required arguments should appear first and we can add optional ones after them. Easier to read this way.