Feature/jss content tokens

[FrankvanZ]

• [x] This PR follows the Contribution Guide
• [x] Changelog updated
• [ ] Upgrade guide entry added

Description / Motivation

This change introduces services for grabbing content tokens, much like dictionary items. This change gets us 1 step closer to (re)introducing the (custom) content token feature in headless development

Testing Details

• [x] Unit Test Added
• [ ] Manual Test/Other (Please elaborate)

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

[yavorsk]

Thanks for the submission @FrankvanZ! I am adding it to our backlog for prioritization and review :)