reftable/dump: support dumping a table's block structure

reftable/dump: support dumping a table's block structure

We're about to introduce new configs that will allow users to have more
control over how exactly reftables are written. To verify that these
configs are effective we will need to take a peak into the actual blocks
written by the reftable backend.

Introduce a new mode to the dumping logic that prints out the block
structure. This logic can be invoked via `test-tool dump-reftables -b`.

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