Filter: operators for working with MAPs

[rpbouman]

          We have a similar opportunity for MAP types:

element_at: https://duckdb.org/docs/sql/functions/map#element_atmap-key
map_contains: https://duckdb.org/docs/sql/functions/map#map_containsmap-key
map_contains_key: https://duckdb.org/docs/sql/functions/map#map_contains_valuemap-value
map_contains_entry(map, key, value)](https://duckdb.org/docs/sql/functions/map#map_contains_entrymap-key-value)

Originally posted by @rpbouman in #318 (comment)

[rpbouman]

Let's hold off on this untill the next DuckDB version drops. I noticed that the JSON reader has some changes which causes the blueworks dataset to return propery structs instead of MAPs, and also the MAP extraction methods have change so as to not return an array anymore. If we build this now we may find we have to redo it again.