Move ConsumerGroup.Manager.start_link/4 to conventional start_link/1

The change is acceptable as `ConsumerGroup` is the only client of
`ConsumerGroup.Manager`. The defaults on `opts` parameter aren't used
and their loss is thus likewise acceptable.

Fixes a compiler warning due to a deprecated usage of
`Supervisor.Spec.worker/3`.

lib/kafka_ex/consumer_group.ex

@@ -363,10 +363,8 @@ defmodule KafkaEx.ConsumerGroup do
     opts = Keyword.put(opts, :supervisor_pid, self())
 
     children = [
-      worker(
-        KafkaEx.ConsumerGroup.Manager,
-        [{gen_consumer_module, consumer_module}, group_name, topics, opts]
-      )
+      {KafkaEx.ConsumerGroup.Manager,
+       {{gen_consumer_module, consumer_module}, group_name, topics, opts}}
     ]
 
     Supervisor.init(children,

lib/kafka_ex/consumer_group/manager.ex

@@ -53,18 +53,18 @@ defmodule KafkaEx.ConsumerGroup.Manager do
 
   @doc false
   # use `KafkaEx.ConsumerGroup.start_link/4` instead
-  @spec start_link(
+  @spec start_link({
           {module, module},
           binary,
           [binary],
           KafkaEx.GenConsumer.options()
-        ) :: GenServer.on_start()
-  def start_link(
+        }) :: GenServer.on_start()
+  def start_link({
         {gen_consumer_module, consumer_module},
         group_name,
         topics,
-        opts \\ []
-      ) do
+        opts
+      }) do
     gen_server_opts = Keyword.get(opts, :gen_server_opts, [])
     consumer_opts = Keyword.drop(opts, [:gen_server_opts])