Read REPLICATE_API_TOKEN from cog context

[aron]

This PR introduces support for the cog context introduced in 0.14.11 into the Replicate SDK. The current_scope helper now makes per-prediction context available via the current_scope().context dict.

A cog model can then provide a REPLICATE_API_TOKEN on a per-prediction basis to be used by the model.

curl -X POST http://localhost/predictions -H 'Content-Type: application/json' -d '{
    "inputs": { "prompt": "make me a sandwich" },
    "context": { "REPLICATE_API_TOKEN": "r8_xyz..." }
}'

def predict(prompt: str) -> str:
    # create a Replicate client using the token provided in the request
    replicate = Replicate()
    output = replicate.run("anthropic/claude-3.5-haiku", {input:
{"prompt": "prompt"}})
    return output