feat: Added Viewer Metadata POST, PUT, and DELETE APIs

[eliasthompson]

Description of the Change

Modifed src\server\api\v1\v1Router.js and src\server\api\v1\controllers\viewersApiController.js to expose POST, PUT, and DELETE APIs for /viewers/:userId/metadata/:metadataKey.

Applicable Issues

N/A

Testing

Manual testing of all three endpoint, using both userId and username.

Screenshots

N/A