Handle MemberIDRequired error in joinGroup to support KIP-394

[sahasan95]

Fixes #1432

What

When a Kafka broker returns MemberIDRequired (error code 79) in response
to a JoinGroup request, the client should retry the request with the
member ID assigned by the broker rather than treating it as a fatal error.

This is required by KIP-394
which was introduced in Kafka 2.2.

Why

Without this fix, consumers connecting to Kafka brokers >= 2.2 may fail
to join a consumer group entirely, resulting in an infinite error loop.

Changes

• In consumergroup.go, after receiving a MemberIDRequired error from
  conn.joinGroup, retry the request with the broker-assigned member ID
  before propagating the error.

Testing

The existing TestConsumerGroupErrors mock-based tests pass with this change.
A full integration test requires a live Kafka broker with KIP-394 support
(Kafka >= 2.2). Happy to add one if you can point me to the right test
infrastructure or provide a test environment setup.