Make explicit relationship between `transVITime` and `epochInfoSlotToUTCTime`

`transVITime` is defined in terms of `epochInfoSlotToUTCTime` in the Alonzo specification. However, both functions are abstract in the Agda specification. This poses the problem of expressing the relationship between the two, which might be relevant in some future proof.

This ticket is to express such relationship in some form of axiom, though probably it will depend on what proof needs to use the relationship.

Start by reading the Alonzo specification's definition of transVITime and the Agda specification's declarations of transVITime and epochInfoSlotToUTCTime. Determine what axiom can express their relationship and what future proof requires; done means the relationship is represented in the specification in a form suitable for that proof.

Written by the indexing model from the issue text.