Fix artifacts v4 backend upload problems (#36805)

* Use base64.RawURLEncoding to avoid equal sign
  * using the nodejs package they seem to get lost
* Support uploads with unspecified length
* Support uploads with a single named blockid
  * without requiring a blockmap

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

tests/integration/integration_test.go

@@ -341,6 +341,10 @@ func MakeRequest(t testing.TB, rw *RequestWrapper, expectedStatus int) *httptest
 	if req.RemoteAddr == "" {
 		req.RemoteAddr = "test-mock:12345"
 	}
+	// Ensure unknown contentLength is seen as -1
+	if req.Body != nil && req.ContentLength == 0 {
+		req.ContentLength = -1
+	}
 	testWebRoutes.ServeHTTP(recorder, req)
 	if expectedStatus != NoExpectedStatus {
 		if expectedStatus != recorder.Code {