The compose now contains multiple image builds, but Weldr API does not
support this feature. Use the first image build every time.
Also start using the new types instead of plain strings.
This commit introduces basic support for upload API. Currently, all the routes
required by cockpit-composer are supported (except for /compose/log).
Also, ComposeEntry struct is moved outside of the store package. I decided
to do it because it isn't connected in any way to store, it's more connected
to API. Due to this move there's currently a known bug that image size is
not returned. This should be solved by moving Image struct inside Compose
struct by follow-up PR.
