kvserver: check L0 sub-levels on allocation

[cockroach-teamcity]

Exalate commented:

cockroachdb/cockroach#78608 --- Release note (ops change): introduce cluster settings kv.allocator.l0_sublevels_threshold and kv.allocator.L0_sublevels_threshold_enforce, which enable excluding stores as targets for allocation and rebalancing of replicas when they have high read amplification, indicated by the number of L0 sub-levels in level 0 of the store's LSM. When both kv.allocator.l0_sublevels_threshold and the cluster average is exceeded, the action corresponding to kv.allocator.l0_sublevels_threshold_enforce is taken. block_none will exclude no candidate stores, block_none_log will exclude no candidates but log an event, block_rebalance_to will exclude candidates stores from being targets of rebalance actions, block_all will exclude candidate stores from being targets of both allocation and rebalancing. Default kv.allocator.l0_sublevels_threshold is set to 20 and kv.allocator.l0_sublevels_threshold_enforce is set to block_none_log.

Jira Issue: DOC-3610