Workload Generator: implement Operation type

[kct949]

Focus on parsing first + make sure to have unit tests for it

[kct949]

This is pretty much done - might be a bit rough around the edges right now + we will change the schema of the operation input models but the gist of it has already been implemented

TODO:

• return values should be specified in some kind of HttpOperationResponsePayloadInput
• returnValue.value should follow the @@ or {{ syntax to specify that this is a dynamically generated value
• MAYBE consider adding a type i.e. response.payload | response.headers | response.statusCode to the ReturnValue model so that you can specify the path like "$$key1.key$$" rather than "$$response.payload.key1.key2$$" ($$ being some arbitrary special character)