Add support for time zone validation

[math-nao]

Enhances #640 .

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

Change Details:

• Add new validator tag called "timeZone" to validate that a value is a valid time zone name
• Add test for this new validator
• Update documentation for this new validator

@go-playground/admins

[elias19r]

Good job!

Looking at baked-in tag names, it seems a lowercase timezone is preferable over timeZone