Answer COPY TO with RETURN_FILES as the count and the list of files written

[tamnd]

Part of #1446.

COPY ... TO now takes RETURN_FILES. When it is on, the copy answers a row with Count as a BIGINT and Files as a VARCHAR[] list of every file it wrote, in the order it wrote them, so a partitioned or rotating copy lists each file. A copy with no rows still lists its one empty file, and with WRITE_EMPTY_FILE false the list is empty.

The option value is cast to BOOLEAN the way the pin reads it, so RETURN_FILES 1 is on, RETURN_FILES false gives the plain changed count, and RETURN_FILES 'x' is refused with the pin's Invalid Input message. Only an option that is on counts as set, and setting both RETURN_FILES and RETURN_STATS is refused with the pin's Binder message. RETURN_STATS on a CSV or JSON copy is refused as not implemented, and the message names the "csv" format for JSON too, as the pin's does.

The writers in export.rs now hand back the files they wrote, so the count and the list come from the same place. Everything here was checked against v2.0.0-dev84237.