[NOGIL] Restrict sharing of Consumer instances across threads

[Ojasva Jain (ojasvajain)]

Introduces a reentrancy gate for the sync Consumer and AIOConsumer that rejects illegal concurrent access to a single Consumer instance from a different caller, while still allowing legitimate re-entrant calls — e.g. a rebalance or commit callback calling back into the Consumer/AIOConsumer that triggered it (on_assign calling assign(), on_commit calling commit(), etc.).

• A unified gate_owner/gate_depth mechanism (Handle_gate_enter()/Handle_gate_exit() in Consumer.c) is shared between both consumer types: the sync Consumer presents its own OS thread ID as the gate identity; AIOConsumer presents a Python-generated logical-caller identity instead, since the "owning" caller may move across ThreadPoolExecutor worker threads between the initial call and any re-entrant calls made from within a callback it triggers.
• Every gated Consumer method (assign, subscribe, poll, consume, commit, close, etc.) was refactored into an _internal entry point that accepts the caller's identity explicitly, plus a thin public wrapper that supplies its own thread ID. AIOConsumer calls the same _internal methods directly with its generated identity, so both consumer types are gated identically in C.
• Backward compatibility: gate enforcement is controlled by CFL_CONSUMER_GATE_ENABLED (confluent_kafka.h) and only applies to Python versions this project has never shipped a wheel for — 3.15+ (any build),
  or 3.14 built free-threaded. On every version already shipped (≤3.14, GIL-based), the gate is compiled out entirely, so existing users see zero behavior change on upgrade.

TODO -> Add unit test cases for AIOConsumer

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