Replace pip cache of session_latest

[javiermtorres]

What's changing

Replaces the /tmp/ray/session_latest/runtime_resources/pip with a link to /tmp/ray_pip_cache. This location is placed in either a separate volume, or shared with the host.

Refs #492 partially

How to test it

Start Lumigator with make local-up. The directory /tmp/ray_pip_cache needs to exist beforehand with 777 permissions. Submit an experiment using the UI. The pip cache should be present in the host directory.

Additional notes for reviewers

N/A

I already...

• Tested the changes in a working environment to ensure they work as expected
• Added some tests for any new functionality
• Updated the documentation (both comments in code and product documentation under /docs)
• Checked if a (backend) DB migration step was required and included it if required
  • No DB changes

[dpoulopoulos]

@javiermtorres looks good to me. I'll run some tests locally and then approve it!

[njbrake]

I didn't test it, but other than a minor comment, looks good to me.

[agpituk]

Looks great! Do you think we can try to warm up this cache in the startup process? not for this PR but as an improvement thing