Introduce the concept of "context" to PredictionRequest (#2330)
This allows a prediction request to include additional per-prediction
context alongside the `input`. This will then be made available to the
predictor via `current_scope().context`. For now this is limited to
`dict[str, str]` but we could expand it later.
The primary benefit is to allow prediction scoped configuration to be
passed alongside user input without changing the input signature.
Usage:
```
POST /predictions
{
"input": { ... },
"context": {
"procedure_source_url": "file:///foo/bar/baz",
"replicate_api_token": "r8_foop",
},
}
```
Then in the predictor:
```
def predict():
context: dict[str, str] = cog.current_scope().context
assert context["foo"] == "bar"
```
