Add a new undocumented parameter to enable the drop of `system.session` collection on a sharded cluster.
The parameter is only meant to be used by specialized operators by following a documented playbook.
To serialize with potential concurrent creations (the collection is forced re-created every 5m) we guarantee to re-use the same shard as coordinator (i.e the first shard alphabetically).
Considering `addShard` or `removeShard` could potentially change the first shard alphabetically, the playbook will warn the operation to

• never enable the parameter if a topology change is running
• never start a topology change if the parameter is enabled
• never enable the parameter during an upgrade/downgrade to prevent doing this operation before 9.1

Notice we could eventually ban topology changes (or fail if running) when enabling the parameters, but this won't be done to simplify the backporting
The drop will be authorized only by `clusterAdmin` roles