ICSHWI-19112: Main API structure. Basic epics-module functionalities

All functionalities used in (epics module)[https://gitlab.esss.lu.se/nice/dev-epics-modules/rmm/-/tree/main] are added in API here. A test program is available under test/, it mocks up a front end node and counts its packets. README is also updated with some basic information.

I tried as hard as I could to avoid the need to initialize the frontends variable explicitly after the FrontEnd instantiation:

rmmAPI.frontends[4][0] = std::make_shared<MockupFE>(&rmmAPI, 4, 0, "myMock");

But at the end it seemed too complex and out of the scope of this branch/ticket to do this.

Closes ICSHWI-19112