Add client certificate (mTLS) support to Configuration

Some HTTP APIs require mutual TLS (mTLS) for authentication, where the client presents a certificate and private key during the TLS handshake. This is the equivalent of curl's `--cert` and `--key` flags:

```bash
$ curl -X GET https://api.example.com/resource \
--cert certificate.pem \
--key private_key.pem
```

Currently, `Configuration` only exposes `verify_tls`, `timeout`, and `follow_redirects`. There is no way to pass a client certificate and key to the underlying Erlang `ssl` module.

One approach would be individual builder functions on `Configuration`, consistent with the existing style:

```gleam
httpc.configure()
|> httpc.client_certificate(certfile: "certificate.pem", keyfile: "private_key.pem")
|> httpc.dispatch(request)
```

Would you consider this as a valid feature request?