Support external files for simulations through API

[steven-cd]

As we move to using the API for more production purposes, the config values are likely to get larger, and I think modellers may want to store them in individual files. The API doesn't really help them to do that though - because it's just the config they'd want to keep in the file, not the whole of the input, they can't just use something like curl -X POST /simulations --data "@file".

We could:

• Add a different endpoint to which people can post the contents of the file (e.g. using --data "@file"), which returns some ID, followed by a call to POST /simulations referencing that ID to trigger the actual simulation
  • This requires us to store the config somewhere temporarily, which we can't do within the web-ui container. One option would be to store it in the Blob Store, which would correspond nicely with one of the options for Error on large payloads from GitHub API #43
• Create a client library (or libraries) that has a convenience method for reading the config from a file
  • There are tools for generating libraries from the OpenAPI spec, and I'd assume we'd then be able to add some convenience. Might be good for other reasons as well, but doesn't have the ease of curl, and obviously has maintenance overhead.