resample_(cube_)spatial: support metadata transform

[jdries]

The cube metadata classes currently do not support resampling the spatial dimension. As a consequence, this metadata is incorrect and misaligned with the actual state on the server.
Server-side, the same problem exists in our 'dry-run' setup: no resampling, so incorrect metadata.

By implementing this in this repo, we can solve both problems at once.
For the user, better client metadata for cubes gives another means to detect issues early.