To enable tablets replication one needs to turn on the (experimental) feature and specify the `initial_tablets: N` option when creating a keyspace. We want tablets to become default in the future and allow users to explicitly opt it out if they want to.
This PR solves this by changing the CREATE KEYSPACE syntax wrt tablets options. Now there's a new TABLETS options map and the usage is
* `CREATE KEYSPACE ...` will turn tablets on or off based on cluster feature being enabled/disabled
* `CREATE KEYSPACE ... WITH TABLETS = { 'enabled': false }` will turn tablets off regardless of what
* `CREATE KEYSPACE ... WITH TABLETS = { 'enabled': true }` will try to enable tablets with default configuration
* `CREATE KEYSPACE ... WITH TABLETS = { 'initial': <int> }` is now the replacement for `REPLICATION = { ... 'initial_tablets': <int> }` thing
fixes: #16319Closesscylladb/scylladb#16364
* github.com:scylladb/scylladb:
code: Enable tablets if cluster feature is enabled
test: Turn off tablets feature by default
test: Move test_tablet_drain_failure_during_decommission to another suite
test/tablets: Enable tables for real on test keyspace
test/tablets: Make timestamp local
cql3: Add feature service to as_ks_metadata_update()
cql3: Add feature service to ks_prop_defs::as_ks_metadata()
cql3: Add feature service to get_keyspace_metadata()
cql: Add tablets on/off switch to CREATE KEYSPACE
cql: Move initial_tablets from REPLICATION to TABLETS in DDL
network_topology_strategy: Estimate initial_tablets if 0 is set
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