Support the DuckDB MAP type

[JelteF]

Description

Postgres does not support a generic MAP type. So we need to make some concessions here. But what we can do is the following:

1. If people actually select a MAP column in the output of their query, we should probably just convert it to its text representation.
2. To allow operations on it like indexing, should define a new Postgres type called duckdb.map and implement the index/subscript operator on it. That subscript operator should then return a duckdb.unresolved_type, similarly to how the index/subscript operator is implemented on duckdb.unresolved_type.

Once we do this we can un-exclude:

pg_duckdb/test/regression/expected/test_all_types.out

Line 18 in 02ebeae

map,