API endpoint for changing/creating/deleting multiple files (#24887)

This PR creates an API endpoint for creating/updating/deleting multiple
files in one API call similar to the solution provided by
[GitLab](https://docs.gitlab.com/ee/api/commits.html#create-a-commit-with-multiple-files-and-actions).

To archive this, the CreateOrUpdateRepoFile and DeleteRepoFIle functions
in files service are unified into one function supporting multiple files
and actions.

Resolves #14619

routers/api/v1/swagger/repo.go

@@ -296,6 +296,13 @@ type swaggerFileResponse struct {
 	Body api.FileResponse `json:"body"`
 }
 
+// FilesResponse
+// swagger:response FilesResponse
+type swaggerFilesResponse struct {
+	// in: body
+	Body api.FilesResponse `json:"body"`
+}
+
 // ContentsResponse
 // swagger:response ContentsResponse
 type swaggerContentsResponse struct {