Add a more fluent API wrapper

[paulo-raca]

Instead of importing lots of different packages, the new "Api" object encapsulates everything neatly:

This builds on top of #688 and #689, and is currently being used in my Serpro API

The simplified usage looks like this:

    client = consulta_cpf.Api(
        "https://gateway.apiserpro.serpro.gov.br/consulta-cpf-df/v1",
        **credentials
    )

    print(
        "Consulta CPF", 
        client.cpf.get_cpf_ni(ni="45317828791"))

TODO:

• Update the documentation

[dbanty]

Moving toward an object-oriented style is a non-goal for this project, as is adding multiple ways to call the functions. There will eventually be a better way to manage clients, but API functions will be imported for the foreseeable future.