[SS-69] Make it obvious: Iceberg sink's AWS connection isn't used.

[ublubu]

For Iceberg sinks to S3 Tables, we've been reusing the Iceberg Catalog connection's AWS creds.
Users separately specify AWS creds for the Iceberg sink itself, but we (validate and then) ignore them.

We could remove the AWS creds option from Iceberg sinks, but:

• Users have already defined Iceberg sinks with superfluous AWS creds (because it's required).
• We'll likely need creds on Iceberg sinks when we support Polaris + <insert storage service here>.

In this PR:

• Add comments about the sink creds being ignored.
• Make sink creds optional. Iceberg sinks to Google BigLake will just reuse the catalog connection's GCP creds.

stacked on #36692