schema: expose functions to retrieve label values

There are currently no constructors other than JSON deserializers (not sure if this is intentional). Any code that will try to construct a new ImageManifest, starting from `&ImageManifest{}`, will have `Labels == nil`. Any code that will set `manifest.Label = nil` (which is legal, as it is an exported field), will also have it set to nil. The `GetLabel` method should not panic with nil dereference in such situation, but return values that signifies that label was not found.