Allow Stumps to simulate/mock an endpoint before the endpoint is available

[nquinn-mw]

Currently, in order to create a stump, we need to perform comms to ultimate real endpoint through stumps server whilst it's recording. Then we'll create a stump from the recording and we're able to customise how the stump is served.

However, what about when the ultimate real endpoint isn't ready yet, but we want to use stumps to simulate possible responses that we expect from the real endpoint?

Currently, when the endpoint doesn't exist for example, we might send a request to the ultimate endpoint and it might respond with a HTTP503/500, etc. In that case, the recording doesn't have a response body, and we're not able to add a response body to customise it (i.e. to stick in the response body that you want sent to your client).
I think it'd be useful if we could add a response body to stump, regardless of whether or not the recording from which the stump was created had a response body or not.