Postgres COPY API improvements

[abonander]

• Export PgCopyIn from sqlx::postgres
  • Documentation for this type is currently not visible but knowing what methods it has is important to understanding how to use it.
• Downgrade &mut self to &self for Pool<Postgres>::{copy_in_raw, copy_out_raw}
  • Signature was likely copied from the same methods on PgConnection, but &mut self shouldn't be required for Pool (backwards-compatible change)

[abonander]

Closed by #1537