[v2] Add support for AssetsHandler (#1325)

* [website] Fix devserver default value doc

* [v2] Add support for AssetsHandler

AssetsHandler is a http.Handler delegate, which gets called
as a fallback for all Non-GET requests and for GET requests
for which the Assets didn’t find the file.

Known Limitations on Linux:
- All requests are GET requests
- No request headers
- No request body
- No response status code, only StatusOK will be returned
- No response headers

Known Limitations on Windows:
-  Request body is leaking memory. Seems to be a bug in
    WebView2, investigation angoing.

Most of these limitations on Linux will be fixed in the future with
adding support for Webkit2Gtk 2.36.0+.

* [v2, linux] Add response streaming support

The complete response won’t be held anymore in memory and will
be streamed to WebKit2.

Co-authored-by: Lea Anthony <lea.anthony@gmail.com>

v2/internal/frontend/devserver/devserver.go

@@ -12,7 +12,6 @@ import (
 	"log"
 	"net"
 	"net/http"
-	"net/http/httputil"
 	"net/url"
 	"strings"
 	"sync"
@@ -84,12 +83,16 @@ func (d *DevWebServer) Run(ctx context.Context) error {
 			return err
 		}
 
+		if externalURL.Host == "" {
+			return fmt.Errorf("Invalid frontend:dev:serverUrl missing protocol scheme?")
+		}
+
 		waitCb := func() { d.LogDebug("Waiting for frontend DevServer '%s' to be ready", externalURL) }
 		if !checkPortIsOpen(externalURL.Host, time.Minute, waitCb) {
 			d.logger.Error("Timeout waiting for frontend DevServer")
 		}
 
-		assetHandler = httputil.NewSingleHostReverseProxy(externalURL)
+		assetHandler = newExternalDevServerAssetHandler(d.logger, externalURL, d.appoptions.AssetsHandler)
 	}
 
 	// Setup internal dev server
@@ -103,7 +106,7 @@ func (d *DevWebServer) Run(ctx context.Context) error {
 		log.Fatal(err)
 	}
 
-	d.server.GET("/*", func(c echo.Context) error {
+	d.server.Any("/*", func(c echo.Context) error {
 		assetServer.ServeHTTP(c.Response(), c.Request())
 		return nil
 	})