Feature: interpolate operator

An operator that based on the upstream value and interpolation config (when to achieve target value, with what easing)

- Should be able to emit interpolated values based on an optional observable input of `interpolate_on`, with the continous/once per frame being the default behavior
- should be able to configure how to react to a new upstream interpolation when the previous one had not finished yet, like continue form the current interpolated value, or from the expected result of the previous one.
- Consider using a separate InterpolatedObservable as the primitive of this operator so that other higher order operators could be used to offer more behaviors like concatenating. Or even formalizing "Task" observables, and build interpolation around that abstraction