koji: use typeinfo in CGImport metadata instead of the legacy format

From Koji Content Generator Metadata[1]:

"maven, win, or image: Legacy build type names which appear at this level
instead of inside typeinfo."

=> see, it's legacy

"typeinfo: A map whose entries are the names of the build types used for
this build, which are free form maps containing type-specific information
for this build."

=> struct{} is used for typeinfo.image because the docs says it should contain
"a free form map", null apparently isn't an option.

[1]: https://docs.pagure.org/koji/content_generator_metadata/

As suggested by the Brew maintainers Tomáš Kopeček and Lubomír Sedlář.

internal/upload/koji/koji.go

@@ -23,8 +23,12 @@ type Koji struct {
 	transport http.RoundTripper
 }
 
+type TypeInfo struct {
+	Image struct{} `json:"image"`
+}
+
 type ImageBuildExtra struct {
-	Image interface{} `json:"image"` // No extra info tracked at build level.
+	TypeInfo TypeInfo `json:"typeinfo"`
 }
 
 type ImageBuild struct {