Memory - Switch to Arduino as ESP-IDF component

In Issues #69 (closed) #70 (closed) we tried Arduino as an ESP-IDF component and the ESP-AT firmware.

• Arduino as ESP-IDF component works and could be merged seamlessly. Still limited to the resources of one ESP32 but seems possible.
• ESP-AT needs ca. a week to adapt our code to AT-commands but is possible too. We have a bit more headroom for development but another mcu adds complexity/error-sources.

Decide what to do and then implement.