OAPI: add endpoint for a pre-flight job check

Add an endpoint that mimicks the job submission endpoint, to see whether
the job survives the job compiler script. This can be used to fail early,
before actually sending files to the farm.

pkg/api/flamenco-openapi.yaml

@@ -650,6 +650,36 @@ paths:
             application/json:
               schema: { $ref: "#/components/schemas/Error" }
 
+  /api/v3/jobs/check:
+    summary: Check the job for validity, without creating it.
+    post:
+      operationId: submitJobCheck
+      summary: Submit a new job for Flamenco Manager to check.
+      tags: [jobs]
+      requestBody:
+        description: Job to check
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/SubmittedJob"
+      responses:
+        "204":
+          description: Job was succesfully compiled into individual tasks. The job and tasks have NOT been stored in the database, though.
+        "412":
+          description: >
+            The given job type etag does not match the job type etag on the
+            Manager. This is likely due to the client caching the job type for
+            too long.
+          content:
+            application/json:
+              schema: { $ref: "#/components/schemas/Error" }
+        default:
+          description: Error message
+          content:
+            application/json:
+              schema: { $ref: "#/components/schemas/Error" }
+
   /api/v3/jobs/query:
     summary: Obtain jobs with filtering and sorting.
     post: