Introduce a versioning system for the api - perhaps semver?

[howard-bailey]

Problem
Currently when changes are made to the game-service (especially breaking changes) this also breaks in the dev environment of the frontend. We need to decouple this, so that the frontend can choose which version to use.

Success Criteria

• Agree on versioning system (perhaps we can use semver's standards)

• When changes are made, release a new version which the frontend can then update to when ready (the frontend will also implement a similar versioning system)..