Capture the COPY ... FROM STDIN payload verbatim

[LucaCappelletti94]

Parsing COPY t (a, b) FROM stdin; with two rows of two columns produced six values with nothing marking where the first row ended, so the rows could not be recovered even by counting columns. Printing the statement did not reproduce its input either, and every print and re-parse cycle inserted one more tab before each field, without bound.

The cause is that the payload was being split on tabs and newlines with \N interpreted, which is a partial implementation of the PostgreSQL text format. This removes that rather than completing it. Statement::Copy now carries payload: Option<String>, holding the text between the command and the \. terminator exactly as written, and Display writes it back unchanged. What a tab or a \N means is left to whoever consumes the statement. None means the statement carried no inline data, so a bare COPY t FROM STDIN; still prints without an invented terminator, while Some("") is an explicitly empty payload.

This breaks Statement::Copy, and Parser::parse_tsv and Parser::parse_tab_value are removed along with the format handling. Every other COPY sub-parser was already private.

My suggestion here is that either we go full csv parsing (which I agree might be a bit much), or no parsing at all (which is what I am suggesting we do in this PR), as the current impl was neither and broken.