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/tailscale.go

@@ -1,6 +1,7 @@
 package integration
 
 import (
+	"io"
 	"net/netip"
 	"net/url"
 
@@ -41,4 +42,6 @@ type TailscaleClient interface {
 	// FailingPeersAsString returns a formatted-ish multi-line-string of peers in the client
 	// and a bool indicating if the clients online count and peer count is equal.
 	FailingPeersAsString() (string, bool, error)
+
+	WriteLogs(stdout, stderr io.Writer) error
 }