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When creating a topic a user may want to know what the broker defaults will be for their topic before creating it. 

 Current State/Workarounds

Create then Edit

A user could create the topic as they see fit. Once it is created they could then inspect the topic to determine it's configs. And then they would be able to alter it to suit their needs. This is problematic because there is a period where your topic is in the incorrect state. E.g., we want a `compact` topic, but the broker default is `delete`. We create a topic with defaults, then read the configs, then update the configs to what we want. In the intervening time a user could have produced a message that is invalid for the topic, i.e. a message without a key.

Know What Broker Configs Are

A user could know, or inspect form the broker, the current broker settings. They would then need to know and map those extant configs to the proper topic configs. There is no chance for inconsistent topic creation, but we are now putting the onus on the user of AdminClient to understand the internal workings of the KafkaConfig. E.g., the mappings from broker config to topic config (defined here), cascading elements like `log.retention.{hours,minutes,ms}`, etc. 

Proposed Changes

We propose allowing `name=null` for a `ConfigResource` with `type=TOPIC`. This will allow us to get the default topic configs from the broker through an already existing path.

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