Introduce "socket_keepalive_enable" configuration property

Dear Taras and Denis,

thank you so much for all of your answers within #665 and for picking up the work on `aiokafka`.

I would like to share a story with yours we had to handle just recently. It's a fun read, but obviously not so funny when running into the respective issues.

It is related to some networking issues people have been experiencing when running on Azure and I reported about all of the nitty gritty details within https://github.com/edenhill/librdkafka/issues/3109 already. Enjoy!

So, while I see that `metadata_max_age_ms` is already configurable in order to adjust to the recommended settings suitable when running on Azure (either when running vanilla Kafka or when connecting to Azure Event Hubs),

https://github.com/aio-libs/aiokafka/blob/48f5df54c90cdfd80d079c20bd9c05663cd43a7b/aiokafka/consumer/consumer.py#L111-L114

there should also be a way to configure `socket_keepalive_enable`, as outlined within at https://github.com/edenhill/librdkafka/issues/3109#issuecomment-714471123.

The recommended settings in the context of our observations on Azure would be
```
socket_keepalive_enable = true
metadata_max_age_ms = 180000
```

So, I am humbly asking to take that into consideration for the upcoming release.

With kind regards,
Andreas.