feat(cpp): add functions related to system, tests for purge_* functions, and consumer group functions (WIP)

[slbotbm]

Which issue does this PR close?

Works toward completion of #2100

Rationale

This PR adds functions related to the system, and tests for previously untested functions.

What changed?

This PR adds:

• The following functions and their tests:
  • get_stats
  • get_me
  • get_client
  • get_clients
  • ping
  • heartbeat_interval
  • snapshot
  • purge_stream (TODO)
  • purge_topic (TODO)
• tests for join_consumer_group and leave_consumer_group (TODO)

The places labelled TODO will be completed after merging of #3046.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

If AI tools were used, please answer:

1. Which tools? (e.g., GitHub Copilot, Claude, ChatGPT) codex
2. Scope of usage? (e.g., autocomplete, generated functions, entire implementation) code generation based on explicit instructions given by me.
3. How did you verify the generated code works correctly? ran the tests, read through and understood the generated code
4. Can you explain every line of the code if asked? yes