Add methods to return a sample number just from IsoCode

[hectorAguero]

Currently, the library does not provide a way to generate a sample phone number when only the IsoCode of the country is available. This creates challenges when trying to set a default number and format it.

Key limitations:

• Unable to access the phoneLength (minimum/maximum) for the country.
• Cannot retrieve the leadingDigits to enable the formatter.

Proposed Solution:
The library should include a way to fill a sample phoneNumber, using the max/min phoneLength and the leadingDigits

• An extension on IsoCode
• A factory constructor in PhoneNumber

PhoneNumber.fromIsoCode(IsoCode.US);

[cedvdb]

What about :

PhoneNumber.example(isoCode: IsoCode.US, type: PhoneNumberType.mobile);

The examples are located here if you are up to implement this https://github.com/cedvdb/phone_numbers_parser/blob/main/lib/src/metadata/generated/metadata_examples_by_iso_code.dart

It should be a matter of getting the appropriate data in the map. Not sure about the cases where an example does not exists.