Backend specific conventions decoding

[shoyer]

Currently, we have a single function xarray.decode_cf() that we apply to data loaded from all xarray backends.

This is appropriate for netCDF data, but it's not appropriate for backends with different implementations. For example, it doesn't work for zarr (which is why we have the separate open_zarr), and is also a poor fit for PseudoNetCDF (#1905). In the worst cases (e.g., for PseudoNetCDF) it can actually result in data being decoded twice, which can result in incorrectly scaled data.

Instead, we should declare default decoders as part of the backend API, and use those decoders as the defaults for open_dataset().

This should probably be tackled as part of the broader backends refactor: #1970

[stale]

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