Cast transformer should handle cast failures somehow

[KristianLyng]

Today, the Cast transfromer will simply fail if Sscanf can't convert a field to the desired value. For many use cases, that's reasonable, but for some schemas, it would be better to set a known value. E.g., if a provider sends "supposedNumber": "null", it might be more desireable to convert that to 0.0 or -1 or something the operator selects than simply failing entirely.

You can still work around this today by converting everything to string in a cast transformer, using the edit transformer to change things like "null" to whatever you want, then cast it back, but it's non-obvious, cumbersome and will make skogul convert everything even when it's not necessary.