Input field placeholder support

[chimit]

Support of the input_field_placeholder keyboard parameter that allows you to set a placeholder into the input field:

$bot->reply('Hi', [
    ...Keyboard::create('keyboard')
        ->addRow(KeyboardButton::create('Button'))
        ->resizeKeyboard()
        ->inputFieldPlaceholder('Hello, world!')
        ->toArray(),
    'parse_mode' => 'HTML',
]);

https://core.telegram.org/bots/api#replykeyboardmarkup