Add more specific parameter type for `setData()` method

[maximal]

FCM message will not be received if array shape is different from array<string, string> (string keys and string values).

For example, this code won’t work:

return FcmMessage::create()
    ->setData([
        'string_key1' => 'string_value1',
        'string_key2' => 'string_value1',
        'string_key3' => 666, // integer value
    ])->...

I propose to specify an array shape for the parameter of setData() method in order to help IDEs to analyse the code and to highlight possible errors preemptively. This will save developers’ time (I personally spent on this issue a couple of investigation hours).