Add internal data model for ValueSet

[m-goggins]

PULL REQUEST

Summary

This PR adds Concept, ValueSet and ValueSetDisplay data models to our constants. Note: these concepts have not yet been implemented but will be in the follow up ticket phdi #2789 at which point the old data model can be deleted.

Related Issue

Fixes # phdi#2760

Acceptance Criteria

Please copy the acceptance criteria from your ticket and paste it here for your reviewer(s) A dataclass for this model has been added to the codebase.

Additional Information

Anything else the review team should know?

Checklist

• [ ] If this code affects the other scrum team, have they been notified? (In Slack, as reviewers, etc.)