Reject unsupported aggregates during anon query validation

[edongashi]

verify_anonymization_requirements in validation.c must be extended to check for currently unsupported aggregates such as SUM, MAX, etc. I think whitelisting is better than blacklisting.

To know which aggregates to allow we can check OidCache.postgres.

[edongashi]

I think this works better while rewriting: https://github.com/diffix/extension-prototype/blob/9112e885cd7fbb31e940eeb604b73e2e465b13df/src/query/rewrite.c#L221-L230

[cristianberneanu]

I think it will be better in the long term to keep query validation, analysis and modification separate.
That way, we'll always have a well understood starting point when we reach the anonymization stage.