Add /operations endpoint to list available operations v2

[Boolean263]

Updated version of #17 that's based on the current master (as of submission). From there:

This is a straightforward change to add a /operations endpoint which can be used to get a list of the available operations on the CyberChef server. The returned object has an attribute for each operation name, with the argument description as its value.

This change also documents this new endpoint in README.md and swagger.yml, and also includes a test case.