Run Jsonnet code

[jdockerty]

The server should expose a route which allows Jsonnet code to be sent to it and then evaluated.

POST /api/run with the body containing a binary encoded Jsonnet snippet which can be loaded into a Jsonnet VM.

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Security consideration

Running arbitrary code seems sketchy, although Jsonnet has no ability to modify a file system or execute network calls, it is hermetic.

Perhaps later we can use some sort of sandbox for evaluation.