Add an example of an ExEx with a shutdown task

[onbjerg]

Describe the feature

The task manager in Reth supports shut down tasks/clean up tasks. We should add a simple example of how to leverage these in an ExEx

Additional context

No response

[kunal365roy]

@shekhirin are you still working on this, or can I give it a shot?

[nkysg]

I think this should move to https://github.com/paradigmxyz/reth-exex-examples/issues.