Pass repo index overlays to configure_ros2

[bluka4]

Provide a client with the possibility of providing repo index overlay files in the configure_ros2 call. This enables clients to use the repo index file (ros2.repos) from the official ROS 2 distribution while providing additional repositories or overriding the original.

The call on the client side will look as follows:

configure_ros2(
    distro = "humble",
    repo_index_overlays = [
        ":more_repos.repos",
    ],
)