feat(gateway): ?filename with download=true

This implements 'attachment' mode triggered then
?filename parameter is accompanied with &download=true

When Content-Disposition: attachment is detected by a modern browser
it will skip rendering and immediately open the "save as" dialog,
making this useful feature for using IPFS gateway as target of
"Download" links on various websites.

Parameter name was suggested in:
ipfs/kubo#4177 (comment)


This commit was moved from ipfs/kubo@fd01acd

gateway/core/corehttp/gateway_handler.go

@@ -261,7 +261,11 @@ func (i *gatewayHandler) getOrHeadHandler(w http.ResponseWriter, r *http.Request
 		urlFilename := r.URL.Query().Get("filename")
 		var name string
 		if urlFilename != "" {
-			w.Header().Set("Content-Disposition", fmt.Sprintf("inline; filename*=UTF-8''%s", url.PathEscape(urlFilename)))
+			disposition := "inline"
+			if r.URL.Query().Get("download") == "true" {
+				disposition = "attachment"
+			}
+			w.Header().Set("Content-Disposition", fmt.Sprintf("%s; filename*=UTF-8''%s", disposition, url.PathEscape(urlFilename)))
 			name = urlFilename
 		} else {
 			name = getFilename(urlPath)