custom clients do not respect Api::default_namespaced

[clux]

Custom clients added in #534 rely on a private Client::new_with_default_ns use in Client::try_default.

This creates a bit of a footgun between two modes that should work the same: Client::try_default and Client::new(similar_service_stack).

Possibly we split the constructor and instead have a moving setter:

impl Client {
    fn with_default_namespace<T: Into<String>>)(mut self, ns: T) -> Self {
        self.default_ns = ns.into();
        self
    }
}

that way we can have the the comparatively easier to read:

    let config = Config::infer().await?;
    let https = config.native_tls_https_connector()?;
    let client = Client::new(
        tower::ServiceBuilder::new()
            .layer(config.base_uri_layer())
            .option_layer(config.auth_layer()?)
            .service(hyper::Client::builder().build(https))
    ).with_default_namespace(config.default_ns);

This makes the config.default_ns a more public facing var, should probably rename it as well to default_namespace.