please make user agent string configurable

[anojht]

Hello Replicate team,

We're using your whisperx containers for transcription services. We pass in audio file urls that need to be fetched.
However, some services reject the request due to the user agent string for the request.

Tracing the code, the request gets made on this line with no way to configure the headers or request before it's made:

cog/python/cog/types.py

Line 383 in 1087c98

resp = requests.get(url, stream=True, timeout=10)