Filtering SQL expressions for Postgres and shapefile

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Currently all the SQL expressions have been tested just on GeoPackages, but we also want to support Postgres and shapefiles by the specification. Prepare and test the code on Postgres layers. (Shape files should work, but needs to be verified)

TODO:
- [ ] single select and multiselect values don't work and crash silently, culprit is `IS` operator, in sqlite it is interpreted the same as `=` however in postgres it is used just to check for `NULL` values ([Sqlite docu](https://www.sqlite.org/lang_expr.html), [Postgres docu](https://www.postgresql.org/docs/current/functions-comparison.html))
- [ ] pass boolean values as `0`/`1` instead of `TRUE`/`FALSE` (shapefile doesn't support the later variant)

Locate the SQL expression filtering code and the existing GeoPackage coverage, then prepare equivalent checks against Postgres layers and shapefile layers. Verify that single-select and multiselect values no longer fail silently, that comparisons use the correct operator for Postgres, and that boolean values are passed as 0/1 for shapefiles.

Written by the indexing model from the issue text.