[Refactor] Unify repo search order by logic (#30876)

have repo OrderBy definitions defined in one place and use a single type
for OrderBy database options

routers/api/v1/repo/repo.go

@@ -107,7 +107,7 @@ func Search(ctx *context.APIContext) {
 	// - name: sort
 	//   in: query
 	//   description: sort repos by attribute. Supported values are
-	//                "alpha", "created", "updated", "size", and "id".
+	//                "alpha", "created", "updated", "size", "git_size", "lfs_size", "stars", "forks" and "id".
 	//                Default is "alpha"
 	//   type: string
 	// - name: order