Feat: connection show all

[ushiboy]

Summary

Add nmcli.connection.show_all() method to get list of all connections using nmcli connection show command.

Motivation

Currently, nmcli.connection() executes nmcli connection to get the connection list.
This PR adds nmcli.connection.show_all() which uses nmcli connection show command, providing an explicit correspondence to the nmcli command and supporting the --active option to filter only active connections.

Changes

New API

Added nmcli.connection.show_all(active: bool = False) -> List[Connection]

Usage:

# Get all connections
connections = nmcli.connection.show_all()

# Get only active connections
active_connections = nmcli.connection.show_all(active=True)