[KIP-482]: Update LeaveGroup API (v1 -> v4)

[Ankith-Confluent]

Upgrades LeaveGroup protocol to v4, adding batch member removal (v3) and tagged fields (v4).

v3 changes

A list of members is returned (supporting static membership from KIP-345), where each member contains:

• member_id: The member ID to remove
• group_instance_id: The static group instance ID (nullable)

v4 changes

• Adopts flexible protocol format with compact arrays, compact strings, and tagged fields for future extensibility.

[confluent-cla-assistant]

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[Copilot]

Pull request overview

This PR upgrades the LeaveGroup protocol from v1 to v4, implementing batch member removal (v3) and flexible protocol format with tagged fields (v4). The changes enable removing multiple group members in a single request and adopt Kafka's modern protocol format for future extensibility.

Key changes:

• Modified LeaveGroup API to accept an array of members instead of a single member ID
• Added support for static group membership via group_instance_id field
• Implemented flexible protocol format with compact arrays and tagged fields for v4

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/rdkafka_request.h	Added rd_kafka_leave_member_t struct and updated LeaveGroupRequest signature to accept member arrays
src/rdkafka_request.c	Updated request builder to handle v3 batch members and v4 flexible format; removed old response handler
src/rdkafka_mock_handlers.c	Enhanced mock handler to process multiple members and return per-member errors in v3+
src/rdkafka_cgrp.c	Updated response handler to parse member-level errors and modified leave logic to use new array-based API

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[Copilot]

Corrected spelling of 'memebr' to 'member'.

Suggested change

	* @brief LeaveGroup memebr identity (for LeaveGroup version 3)
	* @brief LeaveGroup member identity (for LeaveGroup version 3)

[Copilot]

The variable member_cnt is declared but never modified or used meaningfully. Consider removing it or using it to make the code more maintainable if it's intended for future use.