Implemented mDNS

[CommanderRedYT]

This PR implements mDNS for OpenDTU. It adds two services, one for the commonly used _http._tcp service, and one custom one (_opendtu._tcp) for auto-discovery.

It also implements the needed changes for the Web-App.

Flash Statistics:

$ du .pio/build/generic_esp32/firmware.bin

# before: 1512
# after: 1540

This PR was inspired by #695.

[CommanderRedYT]

Note: I have commented out the deinit code for mDNS inside NetworkSettings.cpp. With it commented in, the ESP crashes inside lwip tcp functions. I am unfortunately not able to debug this in detail, so help is required there. Other option would be to maybe add a note that after deactivating mDNS, user has to restart the ESP.

Moved mDNS code out of request handler, could not reproduce crash anymore.

[CommanderRedYT]

I cleaned it up a little bit more. I looked at some other projects that use esp32 and mDNS (like WLED for example), and they all only enable/disable mDNS on boot. This means that every time you'd change this option you would have to reboot.

I never had problems with it when turning it on, so if there are problems I could rewrite it so that enabling will work, but disabling only works on boot. But imo it works fine like this.

[tbnobody]

Would you mind combining the commits into one with git squash?

[CommanderRedYT]

@tbnobody Thanks for the Review, I have resolved all of the issues :)

[tbnobody]

Sorry, just had to add some remarks (it triggered my "Internal Monk")

[CommanderRedYT]

Okay, fixed everything again 👍

[CommanderRedYT]

Btw we should maybe link issue #1428 with this PR

[CommanderRedYT]

@tbnobody Any updates?

[tbnobody]

It's already included in my local branch for several days. Just had no time for a release yet. Will (hopefully) publish it tomorrow or tuesday.

[CommanderRedYT]

It's already included in my local branch for several days. Just had no time for a release yet. Will (hopefully) publish it tomorrow or tuesday.

Awesome, thank you very much! Have a nice evening

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