Support providing streams into mixer via CLI

@IanMagnusson asks

> I'm trying to figure out how to mix using the dolma cli args instead of the config. I want to do something like this but I cant figure out how to index the streams arg correctly:

```shell
dolma mix --streams[0].name "$name"
--streams[0].documents "$input_prefix/$file" \
--streams[0].output.path "$output_prefix/$file" \
--streams[0].output.max_size_in_bytes 1000000000 \
--streams[0].attributes s2orc-eval \
--streams[0].filter.exculde "$@.attributes[?(@.bff_duplicate_paragraph_spans_decontamination && @.bff_duplicate_paragraph_spans_decontamination[0] && @.bff_duplicate_paragraph_spans_decontamination[0][2] >= 1.0)]"
```

We should support this use case. As a stopgap, we should support `echo '{...}' | dolma -c - mix`, i.e. allow passing config through stdin.