Support filtering timezones by country code

[aprat84]

What is the reason for this PR?

Filter creating timezones by country code.
Sometimes it is useful to assign a timezone from same country as the item we are going to create.

• A new feature
• Fixed an issue (resolve #ID)

Author's checklist

• Follow the Contribution Guide
• New features and changes are documented
  Support filtering timezones by country code fakerphp.github.io#67

Summary of changes

Add $countryCode parameter to DateTime::timezone() method, to be able to filter the generated timezone by country.

Review checklist

• All checks have passed
• Changes are approved by maintainer

[pimjansen]

This is a breaking change so we cant accept this on the main stream

[bram-pkg]

@pimjansen this is an @experimental class, so technically we can still change this.

[bram-pkg]

@aprat84 if we manage to accept this, make sure to also change the method signature in the parent interface.

[aprat84]

@bram-pkg done! Also changed the phpdoc in Generator class.

Sorry for my ignorance, but, are you sure it is a BC? The $countryCode parameter is optional. If called as it is now (without parameter) its behavior does not change.

I reused the method cause I thought it would be cleaner, but this could be easily done in a separate method (ie: countryTimezone or timezoneInCountry), so we don't cause BC.