reftable/table: introduce iterator for table blocks

reftable/table: introduce iterator for table blocks

Introduce a new iterator that allows the caller to iterate through all
blocks contained in a table. This gives users more fine-grained control
over how exactly those blocks are being read and exposes information to
callers that was previously inaccessible.

This iterator will be required by a future patch series that adds
consistency checks for the reftable backend. In addition to that though
we will also reimplement `reftable_table_print_blocks()` on top of this
new iterator in a subsequent commit.

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