reftable/stack: expose option to disable auto-compaction

reftable/stack: expose option to disable auto-compaction

The reftable stack already has a variable to configure whether or not to
run auto-compaction, but it is inaccessible to users of the library.
There exist use cases where a caller may want to have more control over
auto-compaction.

Move the `disable_auto_compact` option into `reftable_write_options` to
allow external callers to disable auto-compaction. This will be used in
a subsequent commit.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>