kvnemesis: construct targeted batches

Currently, we have over 60 different client ops in the kvnemsis generator, while the test is configured to choose only 100 random ops per run. Some of these are multi-op batches and transactions that increase the total number of ops run by a given test, but the probability of choosing a fixed op is still pretty low.

To improve our test coverage, we should consider constructing more targeted batches; e.g. read-only batches, write-only batches, etc. We can also consider revising the swarm testing idea prototyped in https://github.com/cockroachdb/cockroach/pull/150495.

Jira issue: CRDB-56185