adapter: Disallow unmaterializable funcs in AS OF queries

[ggevay]

Fixes https://github.com/MaterializeInc/database-issues/issues/10004 by simply disallowing unmaterializable functions in AS OF queries.

Hopefully this is not a thing that users do today, but the PR adds a break glass feature flag that we can flip if it turns out that somebody wants to do this. We can remove the flag after one week in prod.

I think it's good to do this change rather sooner than later, before somebody actually starts doing such queries, at which point it would be more of a hassle to disallow it.

(Doesn't affect SUBSCRIBE; no unmaterializable function calls there anyways.)

[github-actions]

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