Use ESP-IDF only for WiFi

[lptr]

This is a major refactor, and a big step towards dropping Arduino completely.

Fixes #224.

Changes

• The network connection and all network services are now handled via ESP-IDF-only code.
  • Removed MQTTPubSubClient, NTPClient and their dependencies for built-in functions.
  • mDNS doens't use Arduino either.
  • HTTP update works without Arduino, too.
• Logging has been refactored to use the built-in ESP_LOGX() macros. This way logging is significantly simpler, and we get to handle all the system notifications as well in a uniform way (including reporting them over MQTT).
• The firmware works now in Wokwi locally.
• Lots of improvements to heap tracing.
• Many smallimprovements.

One thing that got significantly better with the rewrite is MQTT connections: they happen much faster, and the device is much more responsive. I guess this is because we don't rely on a poll-based mechanism anymore. Yay! 🎉

Most importantly this fixes the memory leak with repeated WiFi reconnections -- see #224. \o/

Known issues

• The code does not work on the MK4 as there is just not enough memory on the ESP32-S2 (it has 320 kB instead of 512 kB on the ESP32-S3). I'm a bit baffled by this, since we've mostly just switched existing functionality provided by Arduino on top of ESP32 to the native implementations, yet we seem to be using more memory, not less. 🤷 I'm hoping that once we drop Arduino completely we'll get back a bunch of said memory.