OAPI: when submitting a job, include the submitter's platform

The submitter's platform is used to perform two-way variable
replacement. The variables of that submitter's platform are looked up,
and their values are replaced with the variable names. This only applies
to the job's settings and metadata, and is only performed on prefixes.

For example, if the submitter's platform has a variable
`render = /shared/frames`, a job setting
`output = "/shared/frames/shot123"` will be stored as
`output = "{render}/shot123".

When a Worker gets a task of this job, `{render}` will be expanded to
the value appropriate for their platform, hence the "two-way" name.

pkg/api/flamenco-openapi.yaml

@@ -1431,11 +1431,25 @@ components:
         "priority": { type: integer, default: 50 }
         "settings": { $ref: "#/components/schemas/JobSettings" }
         "metadata": { $ref: "#/components/schemas/JobMetadata" }
-      required: [name, type, priority]
+        "submitter_platform":
+          type: string
+          description: >
+            Operating system of the submitter. This is used to recognise two-way
+            variables. This should be a lower-case version of the platform, like
+            "linux", "windows", "darwin", "openbsd", etc. Should be ompatible
+            with Go's `runtime.GOOS`; run `go tool dist list` to get a list of
+            possible platforms.
+
+            As a special case, the platform "manager" can be given, which will
+            be interpreted as "the Manager's platform". This is mostly to make
+            test/debug scripts easier, as they can use a static document on all
+            platforms.
+      required: [name, type, priority, submitter_platform]
       example:
         type: "simple-blender-render"
         name: 3Д рендеринг
         priority: 50
+        submitter_platform: linux
         settings:
           blender_cmd: "{blender}"
           filepath: "/render/sf/jobs/scene123.blend"