truncate: add truncate_table transition state

Truncate table for tablets is implemented as a global topology operation.
However, it does not have a transition state associated with it, and
performs the truncate logic in handle_global_request() while
topology::tstate remains empty. This creates problems because
topology::is_busy() uses transition_state to determine if the topology
state machine is busy, and will return false even though a truncate
operation is ongoing.

This change adds a new transition state: truncate_table

service/storage_service.cc

@@ -746,6 +746,8 @@ future<> storage_service::topology_state_load(state_change_hint hint) {
                     [[fallthrough]];
                 case topology::transition_state::left_token_ring:
                     [[fallthrough]];
+                case topology::transition_state::truncate_table:
+                    [[fallthrough]];
                 case topology::transition_state::rollback_to_normal:
                     return read_new_t::no;
                 case topology::transition_state::write_both_read_new:

service/topology_coordinator.cc

@@ -2462,6 +2462,8 @@ class topology_coordinator : public endpoint_lifecycle_subscriber {
                 co_await update_topology_state(std::move(node.guard), {builder.build(), rtbuilder.build()}, str);
             }
                 break;
+            case topology::transition_state::truncate_table:
+                break;
         }
         co_return true;
     };

service/topology_state_machine.cc

@@ -152,6 +152,7 @@ static std::unordered_map<topology::transition_state, sstring> transition_state_
     {topology::transition_state::tablet_draining, "tablet draining"},
     {topology::transition_state::left_token_ring, "left token ring"},
     {topology::transition_state::rollback_to_normal, "rollback to normal"},
+    {topology::transition_state::truncate_table, "truncate table"},
 };
 
 // Allows old deprecated names to be recognized and point to the correct transition.

service/topology_state_machine.hh

@@ -119,6 +119,7 @@ struct topology {
         tablet_resize_finalization,
         left_token_ring,
         rollback_to_normal,
+        truncate_table,
     };
 
     std::optional<transition_state> tstate;