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>

.github/workflows/test-integration.yaml

@@ -41,6 +41,7 @@ jobs:
           - TestResolveMagicDNS
           - TestValidateResolvConf
           - TestDERPServerScenario
+          - TestDERPServerWebsocketScenario
           - TestPingAllByIP
           - TestPingAllByIPPublicDERP
           - TestAuthKeyLogoutAndRelogin