client: expose resource cleanup methods

[ParthSareen]

• added support for context manager protocol
• added explicit .close() method to manage clients

Previously a ResourceWarning was getting thrown:

<sys>:0: ResourceWarning: unclosed <socket.socket fd=3, family=2, type=1, proto=6, laddr=('127.0.0.1', 57073), raddr=('127.0.0.1', 11434)>

This can now be prevented with the client.close() method or by using Python's context manager protocol.
Users can now also use client with context manager protocol as:

with Client() as client:
    messages = [
      {
        'role': 'user',
        'content': 'Why is the sky blue?',
      },
    ]

    response = client.chat('llama3.2', messages=messages)
    print(response['message']['content'])

Closes: #441