`binding-kafka`: add `AlterConsumerGroupOffsets` (API key 53) support

[jfallows]

Summary

Add ALTER_CONSUMER_GROUP_OFFSETS (API key 53) to the KafkaApi enum and
KafkaRequestBeginEx / KafkaResponseBeginEx unions. Required by the
mcp_kafka reset_offsets tool.

This API (introduced in Kafka 2.7) allows an admin client to reset committed
offsets for a consumer group. The group must be inactive (no running members)
at the time of the call — the Kafka broker returns NON_EMPTY_GROUP (error
code 68) if members are still active.

Required changes to kafka.idl

Add to KafkaApi enum:

ALTER_CONSUMER_GROUP_OFFSETS (53)

Add new request and response structs:

struct KafkaAlterGroupTopicPartition
{
    int32 partitionId;
    int64 offset;
    int32 leaderEpoch = -1;
    string16 metadata = null;
}

struct KafkaAlterGroupTopic
{
    string16 name;
    KafkaAlterGroupTopicPartition[] partitions;
}

struct KafkaAlterConsumerGroupOffsetsRequestBeginEx
{
    string16 groupId;
    KafkaAlterGroupTopic[] topics;
}

struct KafkaAlterGroupPartitionResult
{
    int32 partitionId;
    int16 error;
}

struct KafkaAlterGroupTopicResult
{
    string16 name;
    KafkaAlterGroupPartitionResult[] partitions;
}

struct KafkaAlterConsumerGroupOffsetsResponseBeginEx
{
    int32 throttle;
    KafkaAlterGroupTopicResult[] topics;
}

Add to KafkaRequestBeginEx union:

case 53: kafka::stream::KafkaAlterConsumerGroupOffsetsRequestBeginEx alterConsumerGroupOffsets;

Add to KafkaResponseBeginEx union:

case 53: kafka::stream::KafkaAlterConsumerGroupOffsetsResponseBeginEx alterConsumerGroupOffsets;

Notes

• The offset field carries the resolved offset value. Resolution of
  "earliest" and "latest" symbolic offsets to concrete values is the
  responsibility of mcp_kafka — it calls KafkaFetchBeginEx (for latest)
  or uses offset 0 (for earliest) before constructing the
  AlterConsumerGroupOffsets request.
• If NON_EMPTY_GROUP error is returned, mcp_kafka surfaces a clear error
  to the agent: "Consumer group must be inactive before resetting offsets."

Spec test coverage

Add scenarios to specs/binding-kafka.spec/ covering:

• Successful offset reset for a single partition
• Successful offset reset for all partitions on a topic (multiple entries)
• Error: NON_EMPTY_GROUP (error code 68) — group has active members
• Error: GROUP_AUTHORIZATION_FAILED

Additional context

• kafka.idl
• Kafka Protocol — AlterConsumerGroupOffsets