DNSServer refactoring, switch to AsyncUDP

[vortigont]

Following bugfix #7475. Current implementation of DNSServer lib uses WiFiUDP which is not very efficient considering cpu/mem resources. It hooks to loop() for packet processing doing useless malloc's/free each run cycle.

https://github.com/espressif/arduino-esp32/blob/master/libraries/WiFi/src/WiFiUdp.cpp#L205-L221

int WiFiUDP::parsePacket(){
// skipped
  char * buf = new char[1460];
  if ((len = recvfrom(udp_server, buf, 1460, MSG_DONTWAIT, (struct sockaddr *) &si_other, (socklen_t *)&slen)) == -1){
    delete[] buf;
    return 0;
  }

This PR offers refactored DNSServer code based on bundled AsyncUDP lib and some other optimizations.
- got rid of intermediate mem buffers and extra data copies in DNSServer code
- added sanity checks for mem bounds
- optimize label/packet length calculations
- removed some dynamically allocated members
- refactored "Captive Portal" example, now it invokes a pop-up suggesting to access a portal page on majority of modern systems/devices/browsers
- other code cleanup

This PR in a forked lib ESP32-DNSServerAsync that could be easily tested.

Other known issues:

• non A req's ain't handled properly, i.e. always reply with A type records
• pkts with edns opt ain't handled properly, packets ignored due to additional RR's>1

[mrengineer7777]

Exciting work. The challenge with a major change like this is it won't be merged until the next major release (3.0.0?), so #7475 is still useful.

[vortigont]

@mrengineer7777 you maybe right. So I've forked bundled lib integrating this PR into separate repo ESP32-DNSServerAsync for anyone wish to try/test/use.
Feedbacks are welcome ;)

[CLAassistant]

All committers have signed the CLA.

[me-no-dev]

@vortigont could you please sign the CLA?

[vortigont]

@me-no-dev done!

[github-actions]

	Warnings
⚠️	Some issues found for the commit messages in this PR: the commit message "CaptivePortal example refactored": summary looks empty type/action looks empty the commit message "DNSServer code refactoring": summary looks empty type/action looks empty the commit message "DNSServer drop dynamically allocated member structs": summary looks empty type/action looks empty the commit message "DNSServer status getters added": summary looks empty type/action looks empty the commit message "DNSServer use default settings for catch-all setup": summary looks empty type/action looks empty the commit message "DNSServer: labels min length checks, simplified labels parser": type/action should start with a lowercase letter type/action should be one of [change, ci, docs, feat, fix, refactor, remove, revert] the commit message "DNSServer: switch to AsyncUDP instead of WiFiUDP": type/action should start with a lowercase letter type/action should be one of [change, ci, docs, feat, fix, refactor, remove, revert] Please fix these commit messages - here are some basic tips: follow Conventional Commits style correct format of commit message should be: <type/action>(<scope/component>): <summary>, for example fix(esp32): Fixed startup timeout issue allowed types are: change,ci,docs,feat,fix,refactor,remove,revert sufficiently descriptive message summary should be between 20 to 72 characters and start with upper case letter avoid Jira references in commit messages (unavailable/irrelevant for our customers) TIP: Install pre-commit hooks and run this check when committing (uses the Conventional Precommit Linter).
⚠️	Please consider squashing your 11 commits (simplifying branch history).

👋 Hello vortigont, we appreciate your contribution to this project!

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[me-no-dev]

@vortigont thanks a lot!

Tested and confirmed all working as expected