Add type arguments to IConverters and IValidators
IConverters and IValidators required type arguments since Java version 8
(or so). We still use their raw versions such that we get a lot of
Parameterizing these classes would remove a lot of warnings while being trivial to implement. By that, we converge to a point where we could really make use of the warnings.
(from redmine: issue id 4023, created on 2020-05-15)