Make the `MockServer` stoppable

[eudelins-zama]

Currently, once the MockServer is started, it can't be stopped as it runs in a separated tokio task, and its handle is not stored.

It would be nice to have a way to stop the MockServer (with a CancellationToken or using abort on the task handle) to mimic network issues (dropped connection...)

[declark1]

Thanks for the suggestion @eudelins-zama, agreed. Will get this added for the next release.

[eudelins-zama]

Btw, I opened multiple issues on this repo, so let me know if there are some you would like me to help on.

Just let me know a bit more on the "design" decision first (like using CancellationToken vs abort here) and I could try to help 🙂