Add KIP-546 client quota Admin APIs

[Honglei (honglei)]

What

Expose KIP-546 describe_client_quotas() and alter_client_quotas() through the Python AdminClient. This adds the public quota models, C-extension conversion/result handling, type stubs, validation, unit tests, and a Kafka-backed roundtrip test.

This is a stacked draft and depends on confluentinc/librdkafka#5560. It must not be merged until that C API is accepted and the bundled librdkafka revision is updated.

Checklist

• Contains customer facing changes, including new Admin APIs
• Adds unit and integration test coverage

References

• KIP-546: https://cwiki.apache.org/confluence/display/KAFKA/KIP-546%3A+Add+Client+Quota+APIs+to+the+Admin+Client
• librdkafka implementation: [KIP-546] Add client quota Admin APIs librdkafka#5560
• librdkafka feature request: [Feature Request] KIP-546 support librdkafka#5227

Test & Review

Built the extension against the exact reviewed tree from librdkafka#5560, then ran:

• black, isort, and flake8 on changed Python files
• mypy: 119 source files, no issues
• quota/Admin unit suite: 35 passed, 2 unrelated DNS timing tests deselected
• Kafka 4.3.1 integration roundtrip: validate-only, set, describe, and remove

Open questions / Follow-ups

The PR remains draft until librdkafka#5560 lands and this repository updates its librdkafka dependency.

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