BGT ETL: add BGT Lean SQL Script

[justb4]

Besides the full BGT PostGIS version, as a result of the current ETL, there is a need for a BGT PostGIS version and corresposing Dumpfile that has the following characteristics:

• only actual+existing objects (actueelbestaand)
• only tables, no views
• one geometry per table
• only Simple geometries, no Curve/Surface-like
• remove 'cruft-columns', mainly metadata and status-columns, take only relevant columns

This has been applied within geotoko.nl and was named "BGT Lean". See description, data and example dumps here:
https://geocatalogus.nl/dataset/bgt-imgeo-database-dumps

"BGT Lean" is simply an SQL script (file) that is applied to the full BGT latest schema and creates a new schema called bgt_lean. The resulting Dump file is also like 14GB where full BGT PG Dump is 24GB.

BGT Lean is very useful for mapping projects and other tooling which e.g. does not accept 'curve' geometries like
CURVEPOLYGON, COMPOUNDCURVE,CIRCULARSTRING etc. Also one geometry per table solves various issues.

So geotoko.nl is basically open-sourcing the BGT Lean script here to be used and hopefully enhanced by the NLExtract-community.

[justb4]

BGT Lean script is now here:

https://github.com/nlextract/NLExtract/blob/master/bgt/etl/sql/bgt-lean.sql