Provide an API endpoint to validate custom grading scripts

[blerner]

Currently, all the grading script schemas are in the worker/ subdirectory as multiple JSON files. This is inconvenient. It would be better to

(a) consolidate these into a single file,
(b) either make this file available to orchestrator as a static path (e.g. orca.com.invalid/schemas/grading_job.json) so that customers could validate their grading scripts against it, or
(c) add an API to validate a grading job file against the schema (e.g. POST /api/v1/validate_grading_job with an apiKey and a grading script as its JSON payload parameters)

[blerner]

Also important -- the existing .json schema files don't match what the .ts schema-enforcement files are doing. This should be consolidated and fixed.