[KIP-848] Mock handler and Integration tests passing

[emasab]

• Mock handler implementation
• Rename current consumer protocol from generic to classic
• Mock handler with automatic or manual assignment
• More consumer group metadata getters
• Test helpers
• Expedite next HB after FindCoordinator doing it with an exponential backoff to avoid tight loops
• Configurable session timeout and HB interval
• Fix mock handler ListOffsets response LeaderEpoch instead of CurrentLeaderEpoch
• Integration tests passing with AK trunk
• Improve documentation and KIP 848 specific mock tests