[SS-74] Don't try creating a Kafka progress topic if it already exists.

[ublubu]

It is an accepted pattern to create the topics needed for a new sink (e.g. as superuser) before actually creating the sink (as a less-privileged user).

Before this fix, Materialize always tries creating the progress topic, even if it already exists, leading to an authorization failure.
After this fix, Materialize sees that the progress topic exists, so it continues without attempting the operation that requires the "Create" ACL.

[github-actions]

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[ublubu]

This is the new part. The rest is just indented (inside the || { ... })

[ublubu]

Copypaste from https://github.com/ublubu/materialize/blob/588547c24336fc41d5039afce1f5e414a28356d5/src/kafka-util/src/admin.rs#L251-L263

let metadata = client
    .inner()
    // N.B. It is extremely important not to ask specifically
    // about the topic here, even though the API supports it!
    // Asking about the topic will create it automatically...
    // with the wrong number of partitions. Yes, this is
    // unbelievably horrible.
    .fetch_metadata(None, Some(Duration::from_secs(10)))?;
let topic = metadata
    .topics()
    .iter()
    .find(|t| t.name() == new_topic.name)
    .ok_or_else(|| anyhow!("unable to fetch topic metadata after creation"))?;