examples: add monitor_and_probe example for netwatch and portmapper integration

[alpn]

Description

Add a new example demonstrating integration of netwatch and portmapper for monitoring network changes and probing port mapping support on the default route interface.

Breaking Changes

None

Notes & open questions

None

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented. (None)

[flub]

Honestly looking at this example I'm not really sure it explains that much more. I'm mostly wondering about all the various ways of doing the concurrency during the review, there is very little actual clarification about the API. It's all about how to use tokio.

Also, what happens when you start a new probe while one was already in progress?

[alpn]

Honestly looking at this example I'm not really sure it explains that much more. [..] there is very little actual clarification about the API. It's all about how to use tokio.

Well the reason I decided to add this example was because I couldn't figure out how to use this API without reading the code. I'm happy to add this stuff to each crate's respective README as well.

Also, what happens when you start a new probe while one was already in progress?

This shouldn't be possible. A new probe could only be triggered by a network state change, in which case the currently running probe (if any) gets cancelled since our tokio::select! is biased.