[Validator] Added Timezone Constraint Validator

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | []
| License       | MIT
| Doc PR        | yes (symfony/symfony-docs#3907)
This is missing input validation and ignoring of `null`/`""` values. Check LengthValidator for an example.