Improve mosquitto client C++ wrapper to support MQTT v5 features

[martlaak]

• create new mosquittopp_v5 C++ wrapper class for that, to maintain backwards compatibility with old code and not to bloat existing wrapper up with too many redundant callbacks
• make a base class for common code
• clean up and modernize the code a bit

Thank you for contributing your time to the Mosquitto project!

Before you go any further, please note that we cannot accept contributions if
you haven't signed the Eclipse Contributor Agreement.
If you aren't able to do that, or just don't want to, please describe your bug
fix/feature change in an issue. For simple bug fixes it is can be just as easy
for us to be told about the problem and then go fix it directly.

Then please check the following list of things we ask for in your pull request:

• Have you signed the Eclipse Contributor Agreement, using the same email address as you used in your commits?
• Do each of your commits have a "Signed-off-by" line, with the correct email address? Use "git commit -s" to generate this line for you.
• If you are contributing a new feature, is your work based off the develop branch?
• If you are contributing a bugfix, is your work based off the fixes branch?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run make test with your changes locally?

---

[lmussier]

Hi @ralight,
Could you have a look at this ? I might also have another improvment to submit to handle MQTT5 user properties.
Thanks.

[DeuxVis]

Hi @ralight, Could you have a look at this ? I might also have another improvment to submit to handle MQTT5 user properties. Thanks.

Also see #3010