service: add basic cron support

[SMillerDev]

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This adds basic cron support, allowing parts of a cron statement to be either a number or *. After this all service run types are implemented.

[BrewTestBot]

Review period will end on 2021-11-23 at 00:00:00 UTC.

[SMillerDev]

Builds on the functions added in #12460

[MikeMcQuaid]

Looks good so far. Do we expect to support launchd cron equivalents?

[SMillerDev]

https://github.com/Homebrew/brew/pull/12462/files#diff-752349017cbb1770eaf117fc518cbf68da64120fa1e62db02225cbfb95881c26R306 is the only thing that's needed for basic launchd support. Things like ranges and repeated entries will have to wait for the more complicated support.

[MikeMcQuaid]

@SMillerDev Great, thanks!

[BrewTestBot]

Review period ended.