List types

[bjornharrtell]

I initially considered dedicated support for list types but opted out to include them for simplicity.

But there is at least one another way to support them without changing the spec and that would be to formalize column metadata (https://github.com/flatgeobuf/flatgeobuf/blob/master/src/fbs/header.fbs#L55) to be able to say that the columns is to be treated as a list and possibly also contain the possible "domain values". By using such metadata in combination with json column type with expected content to be a json array (or possibly the string type with some separator) the GDAL implementation could be smart about that and convert to nearest possible list representation.