Allow passing arbitrary point-wise and integral functionals to `TimeSeriesCallback`

What about adding the option of using arbitrary functionals of the solution, which could also be integrals like we have in the analysis callback? Then, we could have something like `PointwiseFunctional`s and `IntegralFunctional`s, say, that could use the same infrastructure of the `TimeSeriesCallback`. In the long run, this could get us lift and drag etc.

_Originally posted by @ranocha in https://github.com/trixi-framework/Trixi.jl/issues/556#issuecomment-823338637_

Start by comparing the TimeSeriesCallback with the existing analysis callback infrastructure, since no files or tests are named in the issue. Define the scope for arbitrary pointwise and integral functionals, including how their results should be exposed by the time-series callback, and consider lift and drag as a later use case.

Written by the indexing model from the issue text.