Create a http build source

[nicolaferraro]

/kind feature
/area API

As alternative for the same use case in #505, it may be helpful to provide a http based build source, to allow the user to provide the URL of a package containing the files to start the build from.

[imjasonh]

In the meantime you can specify a ContainerSource which specifies a container to run to fetch and place source into /workspace, or anywhere else you want.

Longer term, this should be an HTTP-fetching Resource that you can use and reuse in a Pipeline. The Resource model makes it a lot easier to define many types of sources, without having to have them all defined by the main repo.