Add `reduce` source for arbitrary temporal reductions

[frazane]

Problem:
The current accumulate source in anemoi-datasets covers the specific case of temporally accumulated fields very well, with a sophisticated logic to determine covering intervals required to compute a target accumulation. However, there is no equivalent source for other temporal reductions, such as mean, max/min, std, etc. Users might need a way to apply other common temporal reductions declaratively in dataset recipes. Concretely, we would currently need this to compute hourly maxima of wind (i.e. wind gust) starting from 10min maxima.

Proposal
Add a new reduce source (or generalise accumulate) to support arbitrary reductions. The interface should mirror accumulate:

input:
  reduce:
    source: {mars: ...}
    period: 6h
    operation: mean  # or max, min, sum, std, etc.
    availability: ...

Importantly, this new source would reuse much of the interval-handling machinery already implemented for the accumulate source.