[humble] Allow Path in substitutions, instead of requiring cast to str (backport #873)

[mergify]

Closes #805
Closes robograph-project/planning#12
Replaces #790
Related to #868

Allow passing pathlib.Path objects directly into contexts expecting a Substitution, for more natural modern pythonic usage. For example IncludeLaunchDescription(Path(__file__).parent / 'file_in_this_dir_launch.yaml'))

As part of this change, stop doing str(Path) casts everywhere it's done for this reason.

---

This is an automatic backport of pull request #873 done by Mergify.

[mergify]

Cherry-pick of 49bcf2d has failed:

On branch mergify/bp/humble/pr-873
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 49bcf2d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   launch/launch/launch_description_sources/python_launch_file_utilities.py
	modified:   launch/launch/some_substitutions_type.py
	modified:   launch/launch/substitutions/path_join_substitution.py
	modified:   launch/launch/utilities/normalize_to_list_of_substitutions_impl.py
	modified:   launch/launch/utilities/typing_file_path.py
	modified:   launch/test/launch/actions/test_include_launch_description.py
	modified:   launch/test/launch/launch_description_source/test_python_launch_description_source.py
	modified:   launch/test/launch/launch_description_source/test_python_launch_file_utilities.py
	modified:   launch/test/launch/substitutions/test_command.py
	modified:   launch/test/launch/substitutions/test_file_content.py
	modified:   launch/test/launch/substitutions/test_path_join_substitution.py
	modified:   launch_pytest/test/launch_pytest/examples/check_node_msgs.py
	modified:   launch_pytest/test/launch_pytest/test_plugin.py
	modified:   test_launch_testing/test/test_launch_testing/actions/test_gtest.py
	modified:   test_launch_testing/test/test_launch_testing/actions/test_pytest.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   launch/launch/substitutions/python_expression.py
	both modified:   launch_xml/test/launch_xml/test_executable.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[emersonknapp]

Pulls: #926
Gist: https://gist.githubusercontent.com/emersonknapp/ce6db14a1b6a5425ec4eb7b9d3f152d0/raw/d03b316805237d76c99584fed96a2b04ace355ab/ros2.repos
BUILD args: --packages-above-and-dependencies launch launch_pytest launch_xml test_launch_testing
TEST args: --packages-above launch launch_pytest launch_xml test_launch_testing
ROS Distro: humble
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/17486

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[ahcorde]

Pulls: #926
Gist: https://gist.githubusercontent.com/ahcorde/b3ba73d6b6bb3e50b1bfd337595a7041/raw/d03b316805237d76c99584fed96a2b04ace355ab/ros2.repos
BUILD args: --packages-above-and-dependencies launch launch_pytest launch_xml test_launch_testing
TEST args: --packages-above launch launch_pytest launch_xml test_launch_testing
ROS Distro: humble
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/17644

• Linux
• Linux-aarch64
• Linux-rhel
• Windows