Implement _fix_agg for exactextract zonal stats

[tm-jc-nacpil]

Fixing the agg spec is not yet implemented, so malformed agg specs result in error (i.e. single dicts that are not in the list). There's an existing implementation for the basic function, but it needs to be tweaked for exactextract as it does not use the "column" field

Existing implementation:https://github.com/thinkingmachines/geowrangler/blob/93c8e126b3198fa15aab432ae927dc5f2cfd8c9f/geowrangler/raster_zonal_stats.py#L48