add impl pointer for ExecutorOptions (backport #2523)

[mergify[bot]]

Follow up of https://github.com/ros2/rclcpp/pull/2505#issuecomment-2059392440, we need this to potentially add more functionality to Jazzy post release, as well it gives us more flexibility to fix bugs in the future.

We already have this for the Executor, so this class is the only one that needs adjustment:

https://github.com/ros2/rclcpp/blob/f7185dc129c40388e66813c96f2ac5dbb73ffe00/rclcpp/include/rclcpp/executor.hpp#L597

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This is an automatic backport of pull request #2523 done by Mergify.

[clalancette]

CI:

• Linux (edit by wjwwood, job was restarted by jenkins)
• Linux-aarch64
• Windows

[clalancette]

Here's a rebuild on Windows, since it failed some tests that I'm not sure it should have:

• Windows