builtin/gc: introduce object database optimization options

builtin/gc: introduce object database optimization options

Introduce `struct odb_optimize_options` to decouple the options that are
specific to optimizing the object database from `struct gc_config`. This
structure will be moved into the object database layer in a subsequent
commit.

Note that there are a small set of backend-specific options in this
structure. In an ideal world those of course wouldn't exist, but as
we're introducing the object database abstractions retroactively we are
somewhat forced to keep them.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>