Make client argument optional in macros kwargs and arguments

[paulo-raca]

This change is useless on its own 😅

So I'm going to give the context instead:
I'm working on a template with a custom API where the client is provided by self._get_client() instead of an explicit argument, which required a few tweaks to those macros:

@attr.s(auto_attribs=True)
class Endpoints:
    _client: Union[Client, Callable[[], Client]]

    def _get_client(self) -> Client:
        client = self._client
        if callable(client):
            client = client()
        return client

    {% for endpoint in endpoint_collection.endpoints %}
    {% set f_name = python_identifier(endpoint.name) %}
    {% set return_string = endpoint.response_type() %}
    def {{f_name}}_detailed(
        self,
        {{ arguments(endpoint, include_client=False) | indent(4) }}
    ) -> Response[{{ return_string }}]:
        {{ docstring(endpoint, return_string) | indent(4) }}

        from . import {{ f_name }} as _endpoint
        return _endpoint.sync_detailed(
            {{ kwargs(endpoint, include_client = False) }}
            client = self._get_client(),
        )
    {% endfor %}

[dbanty]

I don't want to increase the complexity of the existing Jinja code for custom templates—since the usage of custom templates isn't stable. Plus, on the roadmap (somewhere) is changing the way clients work entirely.