The function validate_change in raft_group0_client is used currently to
validate tablet metadata changes, and therefore it applies only to
commands of type topology_change.
But the type mixed_change also allows topology change mutations and it's
in fact used for tablet metadata changes, for example in
keyspace_rf_change.
Therefore, extend validate_change to validate also changes of type
mixed_change, so we can catch issues there as well.
6619e798e7