Generic solution for Implicit Conversions to Interval<T>

[suyashkumar]

The author of https://github.com/google/cql/commit/def266a21cf8fb466434fb226b8a1c4ff7d9f8be did a one-off patch to fix the issue where we were unable to convert FHIR.Period to Interval<T>, even though the modelInfo dictates this conversion. I just wanted to connect the dots here to the underlying more generic issue, which I had identified earlier in convertGeneric and is documented here: https://github.com/google/cql/blob/f4bb613e7ca5324e91bede1ab0eb4ebaa93cd6d5/internal/convert/convert.go#L529-L534

Since https://github.com/google/cql/commit/def266a21cf8fb466434fb226b8a1c4ff7d9f8be didn't mention the b/333923412 bug from there, I just wanted to connect the dots here between that change and the underlying more generic issue to make sure it doesn't slip through the cracks!

[evan-gordon]

Good callout! I was actually reading through some of this code last week in order to fix some generic conversions for lists (waiting on an intern review for that)

I'm not sure how soon I'll be able to dive into this specific issue myself at present, but we'll see.