[`cpf-gen`] Throw error on repeated digits input

[juliolmuller]

Repeated numbers in a CPF is considered invalid, although the calculated check digits indicate it is valid. CPF sequences 111.111.111-11, 222.222.222-22, 333.333.333-33, etc, should be evaluated and denied as valid ones.

Therefore, there are 2 cases that must be handled in cpf-gen package:

• If a 9-digits prefix is provided to CpfGenerator::generate or cpf_gen helper function, it should validate if the input has repeated numbers and throw a runtime error.
• If a sequence of repeated numbers are generated by accident, with or without a provided less-than-9-digits prefix argument, it should rerun the digits generation to avoid invalid outputs.

Unit tests on these two scenarios are required.