Allow chaining of methods for more concise code

[0xPIT]

This change allows chaining of methods for more concise code, similar like in most JavaScript Frameworks. Should not affect previous usage patterns, as the return type was void.

The BasicOTA example could look like this:

 ArduinoOTA
    .onStart([]() {
      
    })
    .onEnd([]() {

    })
    .onProgress([](unsigned int progress, unsigned int total) {

    })
    .onError([](ota_error_t error) {
      
    });