feat AdminClient: support delete_records

[wbarnha]

Hey!

This PR is based on dpkp#2062
It implements delete_records for AdminClient. Thanks to @10101010 for the initial PR.

This PR contains the following improvements on that version:

1. It fixes review comments: adds tests and removes formatting peculiarities
2. It contains an optimization that is important to me: if multiple partitions live on the same broker, it sends one request, instead of one per partition. It is important because I use delete_records in integration tests, and this significantly improves test startup/teardown speed.
3. It checks for errors as much as it can.

I tested it locally with 2.6.0 + py3.9, I'm assuming CI will check for everything else.
Additionally, I would like some direction from maintainers on whether it is possible/required to test this with a multi-broker setup - I couldn't find an example in the tests that uses multiple brokers.

Resolves: dpkp#2344

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This change is