Fix filesystem linking on GCC 8 and add RHEL CI workflow (UniversalRobots#454)

On GCC 8, the <filesystem> implementation is not included in the standard library by default.
It must be explicitly linked via:

`target_link_libraries(urcl PUBLIC stdc++fs)`

Newer GCC versions perform this automatically, which is why the issue
only appears on RHEL 8 buildfarm jobs.

This PR adds the explicit link dependency for `stdc++fs` in the CMake to
ensure the linking for GCC8

To prevent regressions and ensure URCL builds correctly on all supported
RHEL toolchains, this PR introduces a new GitHub Actions workflow:

- Runs URCL builds inside RHEL containers as in
https://github.com.mcas.ms/ros-controls/ros2_control_ci/tree/master?tab=readme-ov-file#ros2_rhel
- Covers multiple ROS2 distributions (Humble, Jazzy, Kilted, Rolling).

The workflow performs a minimal workspace build using colcon inside the
container, matching the RHEL buildfarm configuration.

Author: srvald

.github/workflows/rhel_build.yml

@@ -0,0 +1,33 @@
+name: RHEL Build
+on:
+  workflow_dispatch:
+  pull_request:
+  push:
+    branches:
+      - master
+jobs:
+  rhel_build:
+    name: ${{matrix.ROS_DISTRO.NAME}} (RHEL)
+    runs-on: ubuntu-latest
+    container: ghcr.io/ros-controls/ros:${{ matrix.ROS_DISTRO.NAME }}-rhel
+    env:
+      path: src/${{ github.repository }}
+    strategy:
+      fail-fast: false
+      matrix:
+        ROS_DISTRO:
+          - NAME: humble
+          - NAME: jazzy
+          - NAME: kilted
+          - NAME: rolling
+
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          path: ${{ env.path }}
+      - name: Build workspace
+        shell: bash
+        run: |
+          source /opt/ros/${{ matrix.ROS_DISTRO.NAME }}/setup.bash
+          source /opt/ros2_ws/install/local_setup.bash
+          colcon build --event-handlers console_direct+ --cmake-args -DCMAKE_BUILD_TYPE=RelWithDebInfo

CMakeLists.txt

@@ -84,6 +84,7 @@ else()
     target_compile_options(urcl PUBLIC -fsanitize=address)
     target_link_options(urcl PUBLIC -fsanitize=address)
   endif()
+  target_link_libraries(urcl PUBLIC stdc++fs)
 endif()
 
 target_include_directories(urcl PUBLIC