Updated to use ESP Async WebServer v3+ API

[dkozel]

The returned parameter values are const and need to be indexed using size_t not the implicit int type.

This might be reverse compatible with the v2 API, but I haven't tested that. I also haven't done testing of every API endpoint, but all of the logical flows look unaffected by making the returned parameter const.

I see the large PR #551 which removes ESP Async WebServer, but hope this small change will be useful to allow users to compile with the up-to-date library. There's also a documentation update for the wiki which should be done as the two ESP Async libaries have moved repos, I'll open a PR and link it to this one.

This was tested with Arduino 2.3.4, ESP8266 board package 3.1.2, ESP Async TCP 0b6de755, and ESP Async WebServer v3.6.2.

ESP Async TCP is frozen and will not be further developed according to the maintainer as the ESP8266 is considered mature.

[dkozel]

Realized that the platformio.ini file needed to be updated with the newer dependencies. I also added the esp12a board as I'm using a NodeMCU v3.

[AnvilByte]

I tested this myself and wifi settings isn't being saved, which is a known issue with ESP8266 board package 3.0+
and using 2.6.3 gives me a "error: missing binary operator before token "(" #if __has_include("ArduinoJson.h")" error, so I don't know what's going on