WebAPI: Add resolutions and spatialUnits to mldataset

[clarasb]

This PR:

• is needed for Add indicator of zoomLevel and datasetLevel to map xcube-viewer#544, to access information on the resolutions and spatialUnits of MultilevelDatasets from xcube Server in xcube Viewer.

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[forman]

All fine, but please write a CHANGELOG entry.

Although the code is covered by at least one test, none of them really asserts the correctness of the new settings. Consider writing a new unit test or enhancing one.