Missed mock request callback

[James1x0]

This would solve...

This would give a cleaner look at what requests were not caught by the mockagent during a particular run

The implementation should look like...

Long overdue, as discussed in #1600, we could add a callback to allow users to keep tabs on mismatched requests. I'll be adding a draft implementation soon.

I have also considered...

Another option is to enable a flag to log mismatched requests.

Additional context