db: config: introduce force-gossip-topology-changes

We are going to make the `consistent-topology-changes` experimental
feature unused in 6.0. However, the topology upgrade procedure will
be manual and voluntary, so some 6.0 clusters will be using the
gossip-based topology. Therefore, we need to continue testing the
gossip-based topology. The solution is introducing a new flag,
`force-gossip-topology-changes`, that will enforce the gossip-based
topology in a fresh cluster.

In this patch, we only introduce the parameter without any effect.
Here is the explanation. Making `consistent-topology-changes` unused
and introducing `force-gossip-topology-changes` requires adjustments
in scylla-dtest. We want to merge changes to scylladb and scylla-dtest
in a way that ensures all tests are run correctly during the whole
process. If we merged all changes to scylladb first, before merging
the scylla-dtest changes, all tests would run with the raft-based
topology and the ones excluded in the raft-based topology would fail.
We also can't merge all changes to scylla-dtest first. However, we
can follow this plan:
1. scylladb: merge this patch
2. scylla-dtest: start using `force-gossip-topology-changes`
   in jobs that run without the raft-based topology
3. scylladb: merge the rest of the changes
4. scylla-dtest: merge the rest of the changes

Ref scylladb#17802

db/config.cc

@@ -1122,6 +1122,7 @@ db::config::config(std::shared_ptr<db::extensions> exts)
     , index_cache_fraction(this, "index_cache_fraction", liveness::LiveUpdate, value_status::Used, 0.2,
         "The maximum fraction of cache memory permitted for use by index cache. Clamped to the [0.0; 1.0] range. Must be small enough to not deprive the row cache of memory, but should be big enough to fit a large fraction of the index. The default value 0.2 means that at least 80\% of cache memory is reserved for the row cache, while at most 20\% is usable by the index cache.")
     , consistent_cluster_management(this, "consistent_cluster_management", value_status::Deprecated, true, "Use RAFT for cluster management and DDL.")
+    , force_gossip_topology_changes(this, "force_gossip_topology_changes", value_status::Used, false, "Force gossip-based topology operations in a fresh cluster. Only the first node in the cluster must use it. The rest will fall back to gossip-based operations anyway. This option should be used only for testing.")
     , wasm_cache_memory_fraction(this, "wasm_cache_memory_fraction", value_status::Used, 0.01, "Maximum total size of all WASM instances stored in the cache as fraction of total shard memory.")
     , wasm_cache_timeout_in_ms(this, "wasm_cache_timeout_in_ms", value_status::Used, 5000, "Time after which an instance is evicted from the cache.")
     , wasm_cache_instance_size_limit(this, "wasm_cache_instance_size_limit", value_status::Used, 1024*1024, "Instances with size above this limit will not be stored in the cache.")

db/config.hh

@@ -447,6 +447,7 @@ public:
     named_value<double> index_cache_fraction;
 
     named_value<bool> consistent_cluster_management;
+    named_value<bool> force_gossip_topology_changes;
 
     named_value<double> wasm_cache_memory_fraction;
     named_value<uint32_t> wasm_cache_timeout_in_ms;