Register Beam coder(s) to avoid "Using fallback deterministic coder" warnings #77
Description
Currently, running an Xarray-Beam pipeline generates (irrelevant) warnings about using fallback deterministic coders:
WARNING:apache_beam.coders.coder_impl:Using fallback deterministic coder for type '<class 'xarray_beam._src.core.Key'>' in '[6]: Mean/PerKey/CombinePerKey(MeanCombineFn)/GroupByKey'.
Theoretically, we should be able to explicitly register coders for xarray_beam.Key and tuple[xarray_beam.Key, xarray.Dataset] to turn these off.