raft: add error injection that drops append_entries

It will be needed for a test that simulates the cluster getting stuck
during upgrade. Specifically, it will be used to simulate network
isolation and to prevent raft commands from reaching that node.

service/raft/raft_group_registry.cc

@@ -123,6 +123,10 @@ void raft_group_registry::init_rpc_verbs() {
     ser::raft_rpc_verbs::register_raft_append_entries(&_ms, [handle_raft_rpc] (const rpc::client_info& cinfo, rpc::opt_time_point timeout,
         raft::group_id gid, raft::server_id from, raft::server_id dst, raft::append_request append_request) mutable {
         return handle_raft_rpc(cinfo, gid, from, dst, [from, append_request = std::move(append_request), original_shard_id = this_shard_id()] (raft_rpc& rpc) mutable {
+            if (utils::get_local_injector().enter("raft_drop_incoming_append_entries")) {
+                return;
+            }
+
             // lw_shared_ptr (raft::log_entry_ptr) doesn't support cross-shard ref counting (see debug_shared_ptr_counter_type),
             // so we are copying entries to this shard if it isn't equal the original one.
             rpc.append_entries(std::move(from),