Describe the temporal coverings as TemporalParquet 2.0 struct columns

[estebanzimanyi]

The temporal-covering descriptor states the covering columns of
TemporalParquet 2.0.0. Each covering is a struct column at the root of the
schema, named after its temporal column: bbox is a GeoParquet bounding
box column with the fields xmin, ymin, [zmin,] xmax, ymax[, zmax] in that
order, and tspan and vspan hold the time and value bounds. The
spatial class carries bbox, tspan and the plain srid column, the number
class vspan and tspan, the time-only class tspan.

The parser rejects a bbox whose fields are not a GeoParquet bounding box
column's, in its order, and a class that declares a covering twice. The
projection generator gives, per temporal type, each covering with its
column name and its fields in order, every field with the MEOS expression
that derives it from the value; a binding substitutes VALUE with the
column reference and {col} with its name.