Allow users to mount a cache for models in extractors

[diptanu]

The extractors currently download all the models every time they start which is wasteful

Pass a --cache-dir to indexify extractor command which is visible to the extractor python lib, and allow extractors to use that directory if they wish to cache stuff outside the container.

Each extractor implementation can decide what they want to do with the cache so we are not making decisions on their behalf besides making the directory available.