Add a method to TaskHandle to ensure tokio JoinSets are awaited

[Cictrone]

Even though the tokio JoinSet docs claim

The provided future will start running in the background immediately
when this method is called, even if you don't await anything on this
`JoinSet`.

We are seeing some sort of race condition on faster tomes where this behavior is not occurring(and thus the JoinSet for reporting back output is never executing). To fix this I added a join method for TaskHandle so that way in the Agent.report method we can call it on finished TaskHandles. This should be safe from hanging the thread as is_finished (which is called before) ensures that the pool is empty and the Runtime's JoinHandle is finsihed.

/kind bug
Fixes #754

[Cictrone]

this fix option does hang on pivot.reverse_shell as that function will "complete" but the tokio task will continue for the length of the shell. Thus is_finished returns true even while the shell is alive. as such we wont be merging this solution but will use the one in #924 instead.