Fixes ISO 8601 timestamp validation

The regex used to perform timestamp validation fails to parse valid
timestamps that do not have fractional seconds (e.g. `2020-04-14T19:54:46Z`).

This change (1) updates the regex to account for this and (2) moves the regex
into a global variable so it lives in one place.

The new regex is sourced from Ch 4.7 in `Regular Expressions Cookbook` 2e
by Goyvaerts and Levithan.

note - From what I can tell Postman only allows strings in environment
variables so I cannot simply put the validation function there.