Improve time functions - Githubissues

This PR improves the functions added in https://github.com/smallstep/crypto/pull/534 with some usability changes.

toTime 1719970524 returns a time.Time in UTC from a time or a Unix epoch.
formatTime (now): from a time or Unix epoch returns a time string using RFC3339 format UTC.
parseTime and mustParseTime:
- parseTime: returns the current time.Time in UTC.
- parseTime "2024-07-03T01:30:39Z": parses the given time using RFC3339.
- parseTime "time.UnixDate" "Tue Jul 2 18:31:04 PDT 2024": parses a time using a given format.
- parseTime "time.UnixDate" "Tue Jul 2 18:31:04 PDT 2024" "America/Los_Angeles": parses a time with the given format and time zone.
toTimeLayout returns a layout format from the given name.
- toTimeLayout "RFC3339" returns 2006-01-02T15:04:05Z07:00
- toTimeLayout "time.UnixDate" returns Mon Jan _2 15:04:05 MST 2006

smallstep / crypto