Add utility method to generate build

Adds a utility to generate a build, given a buildConfig and modifies the webhook controller to use the new method.
Not sure what you mean re: mutation in this context, but if this is the pattern that describes the way you can construct valid Builds (just making a &Build{} youself isn't good enough?) it looks like a constructor. Or is this function just one of many future factory methods that can produce a valid Build given a bunch of different disparate sets of inputs? Still trying to understand.