provide `typing.Protocol` for `dash_duo`

Is your feature request related to a problem? Please describe.

The DX for using the pytest fixture dash.testing.plugin.dash_duo() could be improved by providing a typing.Protocol to type hint available methods and their signatures.

Describe the solution you'd like

from typing import Protocol

import dash
from dash.testing.application_runners import import_app
from dash.testing.wait import wait_for_element_by_css_selector, wait_for_text_to_equal


class DashDuo(Protocol):
    "The dash_duo pytest fixture lives in dash.testing.plugin.dash_duo."

    def start_server(self, start_server) -> None:
        ...

    def find_element(self, selector: str) -> dash.development.base_component.Component:
        ...


def test_example_app(dash_duo: DashDuo):  # type hint for dash_duo enables auto-complete
    app = import_app("example")
    dash_duo.start_server(app)

    my_component = dash_duo.find_element("#hello-world")

    assert "my-value" == my_component.get_attribute("value")

    ....

Screenshot from VS Code showing auto-complete in action

Describe alternatives you've considered

None

Start at the dash.testing.plugin.dash_duo fixture and inspect the methods and signatures it exposes, including start_server and find_element. Define a typing.Protocol that represents those fixture methods and verify that annotating a pytest test with it provides the requested method signatures and autocomplete support.