expose rcl shutdown function

[benmaidel]

Made the rcl_shutdown function available.

[hoffmann-stefan]

Looks good as proposed 👍
Would only add a comment about the lifetime of the rcl context, as described in the review comment.

[hoffmann-stefan]

For some background: In our internal codebase we use Microsoft.Extensions.Hosting (see https://learn.microsoft.com/en-us/aspnet/core/fundamentals/host/generic-host?view=aspnetcore-9.0) for our nodes. The method generic host does register for SIGTERM/CTRL-C signal handler, overwriting the one from rcl. So we can use the CancelationToken provided by BackgroundService.ExecuteAsync to react so the shutdown of the node. But as the rcl signal handler is overwritten we need (or should) call rcl_shutdown owerselfs.