Support non-default NpgsqlTypeMappings as element type mappings in the compiled model

[roji]

Npgsql has its own NpgsqlTypeMapping, which allows specifying NpgsqlDbType to be used when sending parameters. Unfortunately, there are some limitations when code-generating the compiled model which make it difficult to set such additional custom values (e.g. NpgsqlDbType) on type mappings; I managed to hack this together when the NpgsqlTypeMapping is a top-level mapping, but it gets too complicated when it's nested as an element type mapping.

/cc @AndriySvyryd

Originally reported in #3362