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>

hscontrol/util/net.go

@@ -13,7 +13,6 @@ func GrpcSocketDialer(ctx context.Context, addr string) (net.Conn, error) {
 	return d.DialContext(ctx, "unix", addr)
 }
 
-
 // TODO(kradalby): Remove after go 1.24, will be in stdlib.
 // Compare returns an integer comparing two prefixes.
 // The result will be 0 if p == p2, -1 if p < p2, and +1 if p > p2.