Make client.ip available before socket connection is established

[falkoschindler]

Description

This change has already been part of PR #2399, which was scheduled for version 2.0.0. But because we could use part of this PR without any breaking changes earlier (PR #3286), we extracted the new client.ip implementation into this issue.

The new implementation might look like this:

client.py

@property
def ip(self) -> Optional[str]:
    """Return the IP address of the client, or None if it is an
    `auto-index page <https://nicegui.io/documentation/section_pages_routing#auto-index_page>`_."""
    return self.request.client.host if self.request is not None and self.request.client is not None else None

test_page.py

def test_ip(screen: Screen):
    @ui.page('/')
    def page():
        ui.label(ui.context.client.ip or 'unknown')

    screen.open('/')
    screen.should_contain('127.0.0.1')