A function called with a flash string, which only has an implementation with `const String&` as argument will be compiled as if it is called with a `String` constructor wrapped around it.

For example this implementation in the .h file:
```c++
bool startsWith(const __FlashStringHelper *prefix) const {
            return this->startsWith(String(prefix));
}
```

This is completely useless as the compiler will generate exactly the same code with or without this function implementation in the .h file.
However if we move the implementation to the .cpp file, this conversion to `String` is only added once in the compiled binary.
In my own project I already managed to shrink the largest (ESP32) build by more than 70k in size (!!) by just adding extra function calls with the conversion in the .cpp file.
This PR is just a simple optimisation which already shrinks a very small build of my project by almost 3k in build size.  (custom_beta_ESP8266_4M1M PIO env of ESPEasy)

```
Flash: [========  ]  82.5% (used 862137 bytes from 1044464 bytes)
Flash: [========  ]  82.3% (used 859545 bytes from 1044464 bytes)
```

Larger builds may benefit even more.