Spatial: Translate EnvelopeCombiner.CombineAsGeometry aggregate function

[roji]

@airbreather graciously added EnvelopeCombiner.Combine for us in NetTopologySuite/NetTopologySuite#610, so once NTS is next released we can add an aggregate method translation for that as a follow-up to #13278:

.NET	SQL Server	SpatiaLite	PostGIS
EnvelopeCombiner.CombineAsGeometry(geoms)	EnvelopeAggregate(@​geoms)	Extent(@​geoms)	ST_Extent(@​geoms)