inconsistent configuration of `begin_offset` for group consumer and consumer

[semarco]

there is a inconsistency in configuring group-consumers and consumers.

looks like the begin_offset is directly on the consumer level options, but should be within the config option, like we configure group consumers.
documentation mentions config if of type :brod.consumer_options(), which includes begin_offset.

 [
  group_consumer: [
    config: [
      begin_offset: :earliest,
      offset_reset_policy: :reset_to_earliest,
      max_bytes: 0
    ]
  ],
  consumer: [
    begin_offset: :earliest,
    config: [
      offset_reset_policy: :reset_to_earliest,
      max_bytes: 0
    ]
  ]
]

should be

[
  group_consumer: [
    config: [
      begin_offset: :earliest,
      offset_reset_policy: :reset_to_earliest,
      max_bytes: 0
    ]
  ],
  consumer: [
    config: [
      begin_offset: :earliest,
      offset_reset_policy: :reset_to_earliest,
      max_bytes: 0
    ]
  ]
]