* feat: add verify client config for embedded DERP
* refactor: embedded DERP no longer verify clients via HTTP
- register the `headscale://` protocol in `http.DefaultTransport` to intercept network requests
- update configuration to use a single boolean option `verify_clients`
* refactor: use `http.HandlerFunc` for type definition
* refactor: some renaming and restructuring
* chore: some renaming and fix lint
* test: fix TestDERPVerifyEndpoint
- `tailscale debug derp` use random node private key
* test: add verify clients integration test for embedded DERP server
* fix: apply code review suggestions
* chore: merge upstream changes
* fix: apply code review suggestions
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Co-authored-by: Kristoffer Dalby <kristoffer@dalby.cc>