feat: qlinear_add

[chachaleo]

Pull Request type

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build-related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: #383

What is the new behavior?

Addition of the feature QLinearAdd

[raphaelDkhn]

LGTM! @chachaleo thanks!

Just four small things that was missing:

• Run scarb run docgen to generate the doc.
• Don't forget to add trait description here. When you run docgen, it will create links in the doc.
• Add operator in compatibility file
• Add operator in Summary file