feat: custom WMTS Capabilities Source

[RobertOrthofer]

Implemented changes

This adds a custom ol-source from capabilities, as described in #602. The source extends the more basic TileImage-source, instead of the wmts-source, as private properties would need to be set in an async manner, as the definition of the source is based on the fetched capabilities document. Such functionality might be interesting for a proper ol-source in the future.

Fixtures have been provided to include a slimmed-down EOx-Capabilities document containing a single layer, as well as a jpeg of the cloudless-2017 layer.

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