Add notation and/or constructors for durations in days, hours, or minutes

[google]

Add notation and/or constructors for durations in days, hours, or minutes

[TristonianJones]

I'm sorry we didn't get back to you on this sooner. The duration format we support within Go, unofficially, is what time.ParseDuration supports: https://golang.org/src/time/format.go?s=40605:40651#L1364

This PR is about making sure the spec and other implementations support the same logic as Go since it seems to be the maximum about of usability with the least amount of confusion.

As for type conversions, we don't make any assumptions about units and offer timestamp + duration overloads, but no overload for duration + int as it's not clear what the intent is of the unit-less number. I don't think it's a super common use case to add two duration values constructed from string inputs, but I could be wrong. The preferred technique would be: duration("2h30m45s") + duration("2s") == duration("8456s")

[slott56]

Are these normalized in any way when a duration is the output from the expression? Would a trivial expression like duration("2h30m45s") output duration("8454s")? Would we need timestamp-like extractor methods for days and weeks? double(duration("8454s").days()) == 0.097847?

Also. What about type conversions? Would these have an assumed unit? duration("2h30m45s") + 2 == duration("8456s") Or would there be no overload for this?

[jsannemo]

This would be very nice to have, and the option of `duration("2h30m45s") over adding each unit separately seems nicer. Both cel-go and cel-cpp supports this, so one might just as well formalize it.

Also, I believe fractional seconds are allowed in both cel-go and cel-cpp (cel-cpp seems to use absl::ParseDuration which afaict is essentially the same logic as time.ParseDuration), but it is not clear that this is the case in the spec.

[JimLarson]

The go syntax seems like a reasonable standard to follow. There doesn't seem to be a standard duration notation in Google SQL or Javascript - the prototype languages we try to follow. There's an ISO-8601 notation for durations, but it's clunky and and not compatible with the current "120s" notation.

[TristonianJones]

A common subset of the duration parsing syntax supported by Go (native), Java (threeten-extra), and C++ (abseil.io) has been introduced in all of the runtimes, and the spec now clarifies the supported notation and character set. Days is not supported, but more granular units are. See #185