Support for In-Memory Retained Messages

[abrauchli]

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Hey Michał,

We needed support for retained messages so there you go. It works but it's not thoroughly tested yet and hasn't seen in-product use. It did fine with 1300 retained 10 char messages but obviously the ESP are not exactly blessed with an abundance of RAM.. I also can't say C++ is my home-turf and it's definitely been a while so I'm happy to take any pointers for things to change. If you don't want it in the code-base that's OK, too. I figured it at least serves as a reference if I open up a PR.

Most of all, thanks for your work on this project! Andy