Handle `COPY OUT` requests over PG Wire

[egormanga]

ClickHouse has an external PostgreSQL database engine. It uses COPY (SELECT … FROM …) TO STDOUT statements to fetch data.

SpacetimeDB can support exporting data to CH just by implementing COPY OUT statements in combination with pgwire's copy module.

Currently, this seems to require an architectural rework of sql—core—pg communication to distinguish between query responses and copies, so I'm not taking on it without discussing the design first.