feat: Add support for custom http clients

[drish]

Introduced a new global level attribute default_http_client, that can be overwritten by users to set their own HTTP Clients. Custom clients need to base off the "interface" HTTPClient defined in resend/http_client.py

Example:

# resend's abstract HTTP Client class.
from resend.http_client import HTTPClient

class CustomRequestsClient(HTTPClient):
    def __init__(self, timeout: int = 30):
        self.timeout = timeout

    def request(
        self,
        method: str,
        url: str,
        headers: Mapping[str, str],
        json: Optional[Union[Dict[str, Any], List[Any]]] = None,
    ) -> Tuple[bytes, int, Dict[str, str]]:
        print(f"[HTTP] {method.upper()} {url} with timeout={self.timeout}")
        try:
            response = requests.request(
                method=method,
                url=url,
                headers=headers,
                json=json,
                timeout=self.timeout,
            )
            return (
                response.content,
                response.status_code,
                dict(response.headers),
            )
        except requests.RequestException as e:
            raise RuntimeError(f"HTTP request failed: {e}") from e


resend.api_key = "re_123"
resend.default_http_client = CustomRequestsClient(timeout=400)

email: resend.Email = resend.Emails.send({...})

closes #145

[graphite-app]

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