refactor: introduce evalengine type and use it

[systay]

Description

We want to improve the typing during planning in the vtgate. A first step in that direction is to create a common type that we can use everywhere. This PR introduces a public type in the evalengine package, and uses it for most type information that we pass around.

Related Issue(s)

Part of solving #14310

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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[vmg]

paired on this with @systay