evalengine: Improve the typing situation for functions

[dbussink]

There are a number of functions like unix_timestamp that have more subtle typing properties and who need additional information. This adds logic to track sizing so we can return the correct type also for those functions.

This removes the last ambiguous typing we have here and fixes a number of queries.

Related Issue(s)

Fixes #14340

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[vitess-bot]

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