kojiapi: include image type exports in Koji job args

Koji image request handling now reads the exports defined by each image
type. All APIs now support reading the exports defined by each image
type. The worker still falls back to "assembler" in case the call comes
from an older version of composer.

internal/worker/json.go

@@ -45,6 +45,7 @@ type KojiInitJobResult struct {
 type OSBuildKojiJob struct {
 	Manifest      distro.Manifest `json:"manifest"`
 	ImageName     string          `json:"image_name"`
+	Exports       []string        `json:"exports"`
 	KojiServer    string          `json:"koji_server"`
 	KojiDirectory string          `json:"koji_directory"`
 	KojiFilename  string          `json:"koji_filename"`