Submissions should have an "assigned Judge"

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As a Judge, I should be able to assign myself to a submission to indicate to other judges that I am in the process of reviewing the submission.

On the submissions service (app/challenges-platform/services/submissions-service.ts), we need to create a new method called "assign" that takes submisisonId (uuid) and judgeId (uuid) as arguments.

On app/challenges-platform/models/Submission.ts submissions should have a nullable/optional attribute called assignee of type Judge.

We will need to create the Judge model in app/challenges-platform/models and create a judges table in db/schema.ts

At the database level, submissions will need a nullable foreign key to the judges table.

For the scope of this ticket, only implement the assign method, do not worry about populating assignee in other submission service methods.