- match paths through a custom AsyncWebHandler instead of using generic not-found fallback handler
- allow MQTT-like patterns when registering paths (`simple/path`, `path/+/something`, `path/#`)
Replaces `relay/0`, `relay/1` etc. with `relay/+`. Magnitudes are plain paths, but using `/+` in case there's more than 1 magnitude of the same type.
- restore `std::function` as callback container (no more single-byte arg nonsense). Still, limit to 1 type per handler type
- adds JSON handlers which will receive JsonObject root as both input and output. Same logic as plain - GET returns resource data, PUT updates it.
- breaking change to `apiAuthenticate(request)`, it no longer will do `request->send(403)` and expect this to be handled externally.
- allow `Api-Key` header containing the key, works for both GET & PUT plain requests. The only way to set apikey for JSON.
- add `ApiRequest::param` to retrieve both GET and PUT params (aka args), remove ApiBuffer
- remove `API_BUFFER_SIZE`. Allow custom form-data key=value pairs for requests, allow to send basic `String`.
- add `API_JSON_BUFFER_SIZE` for the JSON buffer (both input and output)
- `/apis` replaced with `/api/list`, no longer uses custom handler and is an `apiRegister` callback
- `/api/rpc` custom handler replaced with an `apiRegister` callback

WIP further down:
- no more `webLog` for API requests, unless `webAccessLog` / `WEB_ACCESS_LOG` is set to `1`. This also needs to happen to the other handlers. 
- migrate to ArduinoJson v6, since it become apparent it is actually a good upgrade :)
- actually make use of JSON endpoints more, right now it's just existing GET for sensors and relays
- fork ESPAsyncWebServer to cleanup path parsing and temporary objects attached to the request (also, fix things a lot of things based on PRs there...)

Change the underlying command line handling:
- switch to a custom parser, inspired by redis / sds
- update terminalRegisterCommand signature, pass only bare minimum
- clean-up `help` & `commands`. update settings `set`, `get` and `del`
- allow our custom test suite to run command-line tests
- clean-up Stream IO to allow us to print large things into debug stream (for example, `eeprom.dump`)
- send parsing errors to the debug log

As a proof of concept, introduce `TERMINAL_MQTT_SUPPORT` and `TERMINAL_WEB_API_SUPPORT`
- MQTT subscribes to the `<root>/cmd/set` and sends response to the `<root>/cmd`. We can't output too much, as we don't have any large-send API.
- Web API listens to the `/api/cmd?apikey=...&line=...` (or PUT, params inside the body). This one is intended as a possible replacement of the `API_SUPPORT`. Internals introduce a 'task' around the AsyncWebServerRequest object that will simulate what WiFiClient does and push data into it continuously, switching between CONT and SYS.

Both are experimental. We only accept a single command and not every command is updated to use Print `ctx.output` object. We are also somewhat limited by the Print / Stream overall, perhaps I am overestimating the usefulness of Arduino compatibility to such an extent :)
Web API handler can also sometimes show only part of the result, whenever the command tries to yield() by itself waiting for something. Perhaps we would need to create a custom request handler for that specific use-case.