crimson/osd: add crimson_memory config option

Add a crimson_memory configuration option that maps to seastar's
--memory flag, allowing control over the per-OSD seastar memory
allocation. Default is 0, meaning seastar uses its default behavior.

This reduces core dump sizes in test environments (from ~117GB to
~32GB with crimson_memory=32G) by limiting the pre-allocated seastar
memory pool.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>