Dekaf: Collection of small updates

[jshearer]

Description:

• Adds a timeout to Metadata API calls, as we found out that these were hanging indefinitely, and consumers were just opening more and more connections as a result. I'd like to have a more global timeout, but that's complicated by the fact that some requests are potentially long-running, such as Fetch with its max_wait_ms parameter. So I went with a more targeted timeout to start with.
• Skip calls to dekaf::topology::Collection::fetch_spec() if they're not needed -- this reduces the query load on supabase
• Switch from partition selectors using estuary.dev/collection when there is no binding partition selector to name:prefix. Combined with assemble: Switch journal_selector over to using name:prefix instead of estuary.dev/collection #2108 to include name:prefix in partition selectors themselves, this will cause all calls to fetch_partitions to use name:prefix.
  Note: AFAIK, the only case where we won't have a binding partition selector is when a connection is using the old user-token auth which we're getting rid of ASAP. So the change in this PR is only applicable to sessions using user-token auth, of which there are almost none at this point.

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This change is 

[psFried]

LGTM