sql: allow casting interval to mz_timestamp

[maddyblue]

Intervals should also be castable to timestamps since they can be expressed as milliseconds. This is similar to the other casts, and will also be useful for upcoming temporal filter improvements.

Motivation

• This PR adds a feature that has not yet been specified.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:
  • Allow interval types to be cast to mz_timestamp.