Extract challenge management to external microservice

[JohnnyJayJay]

The plan so far has been that the backend (this app) manages challenges it pulls from a source such as git or a plain directory.
For a variety of reasons, it might be nicer to extract this logic to a different service:

• backend focuses on managing games rather than reading, validating, updating challenges
• external service can be used from other contexts:
  • we could use it to validate challenges submitted to our challenge repository
  • other projects that want to make use of open challenge repos could include it.
• work on a separate challenge service can be easily parallelised with backend development

The new challenge microservice should essentially do the following:

• Allow configuration of a "challenge source" - directory with challenges in them (future work: possibly allow multiple sources)
• Upon start up, parse all available challenges and build an index for quick access, containing file names (or some other sort of ID), title, tags, difficulty.
• Watch those directories for changes - when a new challenge is added, add it to the index, when a previous one is updated or deleted, update the index accordingly.
• Provide at least the following endpoints:
  • validate challenge: Takes a challenge as input (including a solution file) and performs the following validation steps:
    • run json schema validation (schema is configurable)
    • validate language and license fields (not very important, can be skipped for now)
    • send the provided solution (if any) and the test case inputs to the code execution server and verify that the solution actually produces the expected results
    • send the test case and example inputs together with the inputFormat field (if present) to the templatespiler server for validation
  • get random challenge: pick a random challenge from the index and send the complete data over.

Additional endpoints that could be useful in the future, but aren't a priority:

• get challenge index (paginated?): get an overview of the available challenges
• get challenge by ID
• search challenges by different criteria
• give challenge feedback: server additionally maintains a persistent storage of feedback for each challenge, containing things like a quality rating, difficulty rating, general comments)

How to ID challenges is still somewhat of an open question when multiple sources are involved. I would just focus on a single-source implementation for now and use file names as IDs.

The new project should probably be AGPL-3.0-or-later, like this one.