PostgreSQL dialect for Lotus.Source.Adapters.Ecto.
Namespaces are PostgreSQL schemas. Query plans come from
EXPLAIN (FORMAT JSON), run inside a read-only transaction.
set_search_path/2 and set_statement_timeout/2 are both honoured.
Read alongside Lotus.Source.Adapters.Ecto.Dialect when writing a dialect
for another SQL engine — this module is the reference implementation.