Add support for multiple tiled vpc overviews

[uclaros]

Allow creating multiple tiled overview files for a vpc.
A new option --overview-length is added to pdal_wrench build_vpc.
The name is modeled after pdal split's and pdal tile's --length parameter, and sets the maximum length/width of overview files to generate.
The decimation logic is preserved (still picking every 1000th point) but the output is tiled on a grid of size overview-length, generating files named <stem>-overview-1.copc.laz etc.
All overviews intersecting a vpc's file are added as extra assets with role overview.

[JanCaha]

Looks good.

Would it be possible, to create simple test, that would just check that after running VPC with the parameter the overviews exist and possibly check for number of overviews generated? So that there is at least some basic smoke test for the functionality? There is a pytest fixture laz_files that will provide list of files for VPC, which you could use.