Add compatibility with only websocket-capable clients (#2132)

* handle control protocol through websocket

The necessary behaviour is already in place,
but the wasm build only issued GETs, and the handler was not invoked.

* get DERP-over-websocket working for wasm clients

* Prepare for testing builtin websocket-over-DERP

Still needs some way to assert that clients are connected through websockets,
rather than the TCP hijacking version of DERP.

* integration tests: properly differentiate between DERP transports

* do not touch unrelated code

* linter fixes

* integration testing: unexport common implementation of derp server scenario

* fixup! integration testing: unexport common implementation of derp server scenario

* dockertestutil/logs: remove unhelpful comment

* update changelog

---------

Co-authored-by: Csaba Sarkadi <sarkadicsa@tutanota.de>

integration/hsic/hsic.go

@@ -461,6 +461,12 @@ func (t *HeadscaleInContainer) Shutdown() (string, string, error) {
 	return stdoutPath, stderrPath, t.pool.Purge(t.container)
 }
 
+// WriteLogs writes the current stdout/stderr log of the container to
+// the given io.Writers.
+func (t *HeadscaleInContainer) WriteLogs(stdout, stderr io.Writer) error {
+	return dockertestutil.WriteLog(t.pool, t.container, stdout, stderr)
+}
+
 // SaveLog saves the current stdout log of the container to a path
 // on the host system.
 func (t *HeadscaleInContainer) SaveLog(path string) (string, string, error) {