[KIP-932] Support callbacks for share consumer python binding

[Kaushik Raina (k-raina)]

Summary

1. Enables error_cb, throttle_cb, log_cb, and oauth_cb on ShareConsumer, which dispatch through the share-consumer drain paths i.e poll, commit_sync, commit_async.
2. Temporarily Rejects stats_cb, statistics.interval.ms, and on_commit at construction time with ValueError. Pre-filter in ShareConsumer_init scans args[0] + kwargs before common_conf_setup.
3. Wires the share-consumer log queue onto rk_rep via rd_kafka_share_set_log_queue(rkshare, NULL) so log records reach the Python logger through the same drain as poll/commit.
4. OAuth init contract: rd_kafka_share_sasl_background_callbacks_enable + wait_for_oauth_token_set so construction returns a ready-to-use client. wait_for_oauth_token_set refactored to no longer own destroy on failure, and each _init now invokes the correct teardown API i.e rd_kafka_destroy vs rd_kafka_share_destroy
5. All callback trampolines now use the librdkafka-supplied rk arg in rd_kafka_yield / rd_kafka_oauthbearer_set_token calls instead of h->rk . Also error_cb additionally tolerates dispatch with no CallState and saves/restores any pending Python exception across the user callback so the constructor's original error isn't silently swallowed.
6. New unit suite tests/test_ShareConsumer_callbacks.py covers dispatch, exception propagation, config rejection, and the CallState wrap on commit paths against an unreachable broker. Integration: throttle_cb under tests/integration/share_consumer/ and full OAUTHBEARER handshake/refresh/auth-failure under tests/integration/share_consumer_oauth/

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