Allow tls_auth_handler to support client side verification only

The tls_auth_handler supports setting up a mTLS connection with the Prometheus push gateway, but I want only to verify the certificate of the server. In other words, I wish to skip setting certfile and keyfile in the tls_auth_handler method when the protocol is not ssl.PROTOCOL_TLS_SERVER.

The authentication on the server side I will handle differently.

I'd be willing to contribute. Should I add it as a conditional to the current tls_auth_handler method (think this will ), or should I create a new tls_handler method?

Add to current tls_auth_handler:

def tls_auth_handler(
        url: str,
        method: str,
        timeout: Optional[float],
        headers: List[Tuple[str, str]],
        data: bytes,
        certfile: str,
        keyfile: str,
        cafile: Optional[str] = None,
        protocol: int = ssl.PROTOCOL_TLS_CLIENT,
        insecure_skip_verify: bool = False,
) -> Callable[[], None]:
    """Handler that implements an HTTPS connection with TLS Auth.

    The default protocol (ssl.PROTOCOL_TLS_CLIENT) will also enable
    ssl.CERT_REQUIRED and SSLContext.check_hostname by default. This can be
    disabled by setting insecure_skip_verify to True.

    Both this handler and the TLS feature on pushgateay are experimental."""
    context = ssl.SSLContext(protocol=protocol)
    if cafile is not None:
        context.load_verify_locations(cafile)
    else:
        context.load_default_certs()

    if insecure_skip_verify:
        context.check_hostname = False
        context.verify_mode = ssl.CERT_NONE
    
    if protocol == ssl.PROTOCOL_TLS_SERVER:
        context.load_cert_chain(certfile=certfile, keyfile=keyfile)
    
    handler = HTTPSHandler(context=context)
    return _make_handler(url, method, timeout, headers, data, handler)

Comienza en el punto de entrada tls_auth_handler y revisa cómo se configura su SSLContext, incluida la construcción de HTTPSHandler. Haz que la verificación exclusiva del cliente funcione sin requerir archivos de certificados del cliente, manteniendo al mismo tiempo la carga de la cadena de certificados para el uso del protocolo del servidor; confirma que el handler sigue admitiendo los modos de verificación y de inseguridad indicados.