Multiple available formats for timestamp flag

[horockey]

Checklist

• Are you running the latest v3 release? The list of releases is here.
• Did you check the manual for your release? The v3 manual is here.
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Problem

Hi!
Using lib I found inconvenient moment: only possible time format, but often it is more convenient and intuitive to set time in shortened formats (ex. 1:06PM instead of '2024-08-06 13:06:00Z').

In v3 it can be solved by writing flag using value with custom Set() AND this value can be gotten via Command.Timestamp(), unlike in v2, where context.Timestamp() casts value to concrete type.

But I suppose it may be a common problem and it is better to feat this feature into lib instead of solving it locally with custom realizations.

Solution suggestion

I suggest to add multiple available time formats for flag and try to parse given value by each of them until parse is successful.
See details in PR