reftable-backend: implement 'optimize' action

reftable-backend: implement 'optimize' action

To make the new generic `optimize` API fully functional, provide an
implementation for the 'reftable' reference backend.

For the reftable backend, the 'optimize' action is to compact its
tables. The existing `reftable_be_pack_refs()` function already provides
this logic, so the new `reftable_be_optimize()` function simply calls
it.

Wire up the new function to the `optimize` slot in the reftable
backend's virtual table.

Mentored-by: Patrick Steinhardt <redacted>
Mentored-by: shejialuo <redacted>
Signed-off-by: Meet Soni <redacted>
Signed-off-by: Junio C Hamano <redacted>