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>

CHANGELOG.md

@@ -1,6 +1,10 @@
 # CHANGELOG
 
-## 0.23.0 (2023-09-18)
+## Next
+
+- Improved compatibilty of built-in DERP server with clients connecting over WebSocket.
+
+## 0.23.0 (2024-09-18)
 
 This release was intended to be mainly a code reorganisation and refactoring, significantly improving the maintainability of the codebase. This should allow us to improve further and make it easier for the maintainers to keep on top of the project.
 However, as you all have noticed, it turned out to become a much larger, much longer release cycle than anticipated. It has ended up to be a release with a lot of rewrites and changes to the code base and functionality of Headscale, cleaning up a lot of technical debt and introducing a lot of improvements. This does come with some breaking changes,