Make use of an OverviewStrategy in the DEMRasterSources implementation

[pomadchin]

At this point DEMRasterSources don't use OverviewStrategy and rely only on the PDAL EPTReader capabilities https://github.com/PDAL/PDAL/blob/2.1.0/io/EptReader.cpp#L293-L318.

This task is to improve control over it to improve real time performance.

The core use case is to be able to select the actual resolution we're querying from the EPT tree.

• Use OverviewStrategy to select the PDAL resolution
• Treat the RasterSource cellSize as request into GT OverviewStrategy given resolutions computed from metadata
• Use returned resolution as query parameter to PDAL Pipeline

Requires: #69

[echeipesh]

Change in EPTRasterSourceProvider

Use case:
We get a WCS request is for 30x30m resolution, we want to query the EPT through PDAL pipeline at at 60x60m resolution:

ept://path/to/catalog?overviewStrategy=auto-2