Add duration helper functions to Sprig date functions

[aeroyorch]

Context

While working on improving duration handling in Helm templates, we identified a set of duration helper functions that would be useful.

An initial proposal was made in Helm repository: helm/helm#31695

During that discussion, it was suggested that these helpers would fit better in Sprig, as it is already used by Helm and the functionality is not Helm-specific.

Proposed functions

Duration parsing:

• mustToDuration: Parse a duration string (e.g. "5m", "2h30m", or seconds like "1", "1.5", etc.) into a duration value, failing on error.

Duration unit extractors:

• durationSeconds
• durationMilliseconds
• durationMicroseconds
• durationNanoseconds
• durationMinutes
• durationHours
• durationDays
• durationWeeks

These functions would return the duration expressed in the corresponding unit (typically as an integer or float, depending on Sprig conventions).

Duration manipulation

• durationRoundTo: Round a duration to a specified duration (time.Duration.Round function).
• durationTruncateTo: Truncate a duration to a specified duration (time.Duration.Truncate function).

Status

This issue is intended as a discussion starter. The function names and exact semantics can be adjusted to align with Sprig’s conventions and maintainers’ preferences. Also if the proposal is ok, I could implement these functions and submit a PR.

Thanks!