Add interpolation observers without overhead of interpolation framework

[nilsvu]

It would be helpful to have interpolation events like #3688 that do simple interpolations (e.g. to a point or line) without the overhead that comes with the interpolation framework. It's quite complicated to add interpolation targets with the framework, involving adding structs to the metavariables, and it comes with options in the input file that are split between the "InterpolationTargets" section and the "EventsAndTriggers". It also involves either adding the interpolator parallel component, or using InterpolateWithoutInterpComponent, which is also quite complicated.
However, it's possible and actually quite straightforward to interpolate to points just within an event and write the result to a file, all contained within the event hpp/cpp, see #3688. The complicated features of the interpolation framework, which was built for dynamic horizon finding, are not needed for this. I propose we add simple observers, each contained within their event hpp/cpp, that interpolate to a point, a line, a sphere, etc. We can do this in post processing using spectre.IO.Exporter.interpolate_to_points, but it would be helpful to do this during simulations to reduce the data written out to disk (or to be able to measure these quantities at higher frequency).