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.
