feat: Async Support with httpx 🔥

[drish]

Introduces async support using the httpx library.

SDK async version

1. Users will need to install the async version of the sdk (extra-requries) with: pip install resend[async]

2. Set the default_http_client:

import resend
# Set up async HTTP client
resend.default_http_client = resend.HTTPXClient()

3. call the async version method (suffixed with _async):

email: resend.Email = await resend.Emails.send_async(params)

[pedroimpulcetto]

wow, I loved this Async support!!! great job @drish