upload/koji: add support for uploading to Koji

Introduce a target for Koji and hooked it up in the worker, so if koji
target is specified, the image is uploaded to koji.

[teg: use system kerberos config rather than reading from env]

internal/target/koji_target.go

@@ -0,0 +1,13 @@
+package target
+
+type KojiTargetOptions struct {
+	Filename        string `json:"filename"`
+	UploadDirectory string `json:"upload_directory"`
+	Server          string `json:"server"`
+}
+
+func (KojiTargetOptions) isTargetOptions() {}
+
+func NewKojiTarget(options *KojiTargetOptions) *Target {
+	return newTarget("org.osbuild.koji", options)
+}

internal/target/target.go

@@ -70,6 +70,8 @@ func UnmarshalTargetOptions(targetName string, rawOptions json.RawMessage) (Targ
 		options = new(AWSTargetOptions)
 	case "org.osbuild.local":
 		options = new(LocalTargetOptions)
+	case "org.osbuild.koji":
+		options = new(KojiTargetOptions)
 	default:
 		return nil, errors.New("unexpected target name")
 	}