system_keyspace: add cleanup_candidate to CDC_GENERATIONS_V3

In the following commits, we implement a garbage collection for
CDC_GENERATIONS_V3. The first step is introducing the clean-up
candidate. It will be continually updated by the CDC generation
publisher and used to remove obsolete data.

db/system_keyspace.cc

@@ -281,6 +281,11 @@ schema_ptr system_keyspace::cdc_generations_v3() {
              * range when the generation was first created. Together with the set of streams above it fully
              * describes the mapping for this particular range. */
             .with_column("ignore_msb", byte_type)
+            /* The identifier and timestamp of the current clean-up candidate - the next generation to be
+             * removed. If set, the candidate will be removed together with all older generations when it
+             * becomes obsolete. Otherwise, the next published CDC generation will become a new candidate.
+             * This process prevents the CDC generation data from endlessly growing. */
+            .with_column("cleanup_candidate", cdc_generation_ts_id_type, column_kind::static_column)
             .with_version(system_keyspace::generate_schema_version(id))
             .build();
     }();