Add read-only config attribute to Consumer, Producer, ShareConsumer

[Paradox (i-am-paradox)]

This adds a read-only config attribute to Consumer, Producer and ShareConsumer, as requested in #465. The AIO wrappers keep their own composition style and are not touched.

The attribute returns a dict of the effective configuration of the instance: it is built from rd_kafka_conf() on the client handle followed by rd_kafka_conf_dump(), so it shows the values actually in use, including librdkafka defaults applied on top of the dict passed to the constructor. The dict is a fresh copy on every access, mutating it does not affect the client.

Two notes on the behaviour:

• Properties that carry internal non-string values in librdkafka's dump are excluded, since they are implementation details of the bindings: the various *_cb pointers set by the bindings, opaque, socket_cb and open_cb. Callbacks passed by the application in the config dict never appear in the dump anyway.
• Values are strings, that is what rd_kafka_conf_dump() returns. client.id comes back as the configured string, session.timeout.ms as '1000' and so on. I kept them as-is rather than trying to parse known numeric keys, the round-trip through int() looked like more guessing than value, but I can convert the well-known numeric properties if that's preferred.

bootstrap.servers can come back as None in the dump when it was consumed into the instance, that is how librdkafka reports it, I left the value untouched.

Tests are in tests/test_misc.py and run without a broker: reading the config of a Producer and a Consumer and checking configured values against the dump, checking that a librdkafka default (api.version.request) is present, that the dict is a detached copy, that the attribute has no setter, and that error_cb passed in the config dict does not show up.

On this machine the unit test files give the same result before and after this change (36 pre-existing failures in the AIO tests, they need dependencies I don't have set up locally), with the 4 new tests passing on top.

Fixes #465

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