Misleading handling of begin offset for Consumer groups

[gdwoolbert3]

Hello brod maintainers! I noticed a slight inconsistency/oversight in how the begin offset is resolved for consumer groups.

The docs and type spec would lead me to believe that the begin_offset in the consumer config should be treated as a :brod.offset_time(). The atom values (:earliest and :latest) seem to be handled correctly. However, if you try to use a :brod.msg_ts() type value, it is handled as a resolved offset, not a timestamp.

Obviously, not using :earliest or :latest is a niche use case but, in that case, handling it as a timestamp seems to be the desired behavior.

This is fairly easy to recreate, using the following child_spec. Everything behaves as expected in this case but, if you attempt to use a timestamp for the begin_offset field of the consumer config, you'll notice it's treated as an offset and NOT a timestamp:

def child_spec(_arg) do
  config = %{
    client: :kafka_client,
    group_id: "consumer_group",
    topics: ["test_topic_1"],
    cb_module: __MODULE__,
    consumer_config: [begin_offset: :earliest],
    init_data: [foo: "bar"],
    message_type: :message_set,
    group_config: [
      offset_commit_policy: :consumer_managed,
      offset_commit_interval_seconds: 5,
      rejoin_delay_seconds: 60,
      reconnect_cool_down_seconds: 60
    ]
  }

  %{
    id: __MODULE__,
    start: {:brod_group_subscriber_v2, :start_link, [config]},
    type: :worker,
    restart: :temporary,
    shutdown: 5000
  }
end

Not a super big deal but it would be nice to have this functionality (or at least update the spec to reduce future confusion)