Add ELO as completion criteria option for curriculum lessons

[jrupert-unity]

Proposed change(s)

-Add Elo as a measure for completion. -Update CI tests accordingly.

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

https://jira.unity3d.com/browse/MLA-2313

Types of change(s)

• [ ] Bug fix
• [x] New feature
• [ ] Code refactor
• [ ] Breaking change
• [x] Documentation update
• [ ] Other (please describe)

Checklist

• [x] Added tests that prove my fix is effective or that my feature works
• [x] Updated the changelog (if applicable)
• [x] Updated the documentation (if applicable)
• [ ] Updated the migration guide (if applicable)

Other comments