admin: Add optional /debug/pprof/profile endpoint

This change introduces a new, optional /debug/pprof/profile endpoint to
the proxy's admin server. This endpoint is feature-gated and, initially,
it will not be included in release builds.

When the feature is enabled, profiling requests are only permitted over
the loopback interface.

The development Dockerfile is updated to prevent stripping debug
symbols so the pprof data is hydrated with debug symbols.

.github/workflows/check-all.yml

@@ -28,3 +28,4 @@ jobs:
       - run: git config --global --add safe.directory "$PWD" # actions/runner#2033
       - run: just fetch
       - run: just check --exclude=linkerd-meshtls-boring
+      - run: just check --exclude=linkerd-meshtls-boring --features=pprof