Add Api::default_namespaced

[kazk]

This is the cleanest I can think of at the moment.

• Client gets the default namespace from Config (most users)
• Client with custom Service defaults to "default", but can specify explicitly with Client::new_with_default_ns(service, default_ns)

Supersedes #413

[clux]

Yeah, this is much nicer 👍 . I'm not sure we even need to export new_with_default_ns, because if people have the namespace they want out of band, they can probably just use Api::namespaced down the line.

[clux]

released in 0.56, thanks again!