[test] introduce Nexus test framework and platform (#10533)

This commit introduces a new test framework named Nexus. The
framework includes the Nexus platform implementation that emulates
platform behavior, allowing multiple nodes running the OpenThread
core stack to be simulated and interact with each other within the
same process.

Unlike the simulation platform, where nodes run in separate processes
and interact via POSIX sockets, Nexus nodes are simulated within a
single process. Nexus tests can interact directly with the C++ or C
OT core APIs, providing more control than the simulation platform's
CLI-based interactions. The flow of time in Nexus tests is directly
controlled by the test itself, allowing for quick time interval
advancement.

This model allows for faster and more scalable simulations, enabling
quick simulation of larger networks for longer durations.

This commit introduces the basic platform implementation, including:
- `nexus_alarm`, `nexus_radio`, and `nexus_settings` modules.
- Logging support, allowing logs to be distinguished per emulated
  node.

.github/workflows/toranj.yml

@@ -190,6 +190,30 @@ jobs:
       run: |
         ./tests/toranj/build.sh posix-15.4
 
+  nexus:
+    name: nexus
+    runs-on: ubuntu-20.04
+    steps:
+    - name: Harden Runner
+      uses: step-security/harden-runner@17d0e2bd7d51742c71671bd19fa12bdc9d40a3d6 # v2.8.1
+      with:
+        egress-policy: audit # TODO: change to 'egress-policy: block' after couple of runs
+
+    - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+      with:
+        submodules: true
+    - name: Bootstrap
+      env:
+        GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+      run: |
+        sudo apt-get update
+        sudo apt-get --no-install-recommends install -y clang-10 clang++-10 ninja-build llvm lcov
+        sudo apt-get --no-install-recommends install -y g++-multilib
+    - name: Build & Run
+      run: |
+        ./tests/nexus/build.sh
+        ninja test
+
   upload-coverage:
     needs:
     - toranj-cli