Allow "all" as a feature flag name

`rabbitmqctl enable_feature_flag all` enables all currently disabled feature flags.
While rabbitmq/rabbitmq-cli#455 asked for
`--all`, I decided to just use "all" for consistency with
`rabbitmq-queues rebalance all`.

Closes rabbitmq/rabbitmq-cli#455

(cherry picked from commit af9e877)

deps/rabbitmq_cli/lib/rabbitmq/cli/ctl/commands/enable_feature_flag_command.ex

@@ -17,6 +17,16 @@ defmodule RabbitMQ.CLI.Ctl.Commands.EnableFeatureFlagCommand do
 
   use RabbitMQ.CLI.Core.RequiresRabbitAppRunning
 
+  def run(["all"], %{node: node_name}) do
+    case :rabbit_misc.rpc_call(node_name, :rabbit_feature_flags, :enable_all, []) do
+      # Server does not support feature flags, consider none are available.
+      # See rabbitmq/rabbitmq-cli#344 for context. MK.
+      {:badrpc, {:EXIT, {:undef, _}}} -> {:error, :unsupported}
+      {:badrpc, _} = err    -> err
+      other                 -> other
+    end
+  end
+
   def run([feature_flag], %{node: node_name}) do
     case :rabbit_misc.rpc_call(node_name, :rabbit_feature_flags, :enable, [String.to_atom(feature_flag)]) do
       # Server does not support feature flags, consider none are available.
@@ -32,11 +42,13 @@ defmodule RabbitMQ.CLI.Ctl.Commands.EnableFeatureFlagCommand do
   end
   use RabbitMQ.CLI.DefaultOutput
 
-  def usage, do: "enable_feature_flag <feature_flag>"
+  def usage, do: "enable_feature_flag < all | feature_flag >"
 
   def help_section(), do: :feature_flags
 
   def description(), do: "Enables a feature flag on target node"
 
+  def banner(["all"], _), do: "Enabling all feature flags ..."
+
   def banner([feature_flag], _), do: "Enabling feature flag \"#{feature_flag}\" ..."
 end

deps/rabbitmq_cli/test/ctl/enable_feature_flag_test.exs

@@ -57,6 +57,12 @@ defmodule EnableFeatureFlagCommandTest do
     assert list_feature_flags(:enabled) |> Map.has_key?(context[:feature_flag])
   end
 
+  test "run: enabling all feature flags succeeds", context do
+    enable_feature_flag context[:feature_flag]
+    assert @command.run(["all"], context[:opts]) == :ok
+    assert list_feature_flags(:enabled) |> Map.has_key?(context[:feature_flag])
+  end
+
   test "banner", context do
     assert @command.banner([context[:feature_flag]], context[:opts])
       =~ ~r/Enabling feature flag \"#{context[:feature_flag]}\" \.\.\./