[8.x] Expose the underlying output implementation for OutputStyle

[jhdxr]

As requested and discussed in laravel/ideas#2064, some symfony features (appendRow for tables, or multi progress bar) requires output implementation other than BufferedOutput. However, since laravel hides the $output, there is no way for users to do so unless rewrting everything related to console output.

There was an attemp (#31426) to support it but it broke many tests and got reverted finally (4094d78), because that PR also changes the consturction method of Table.

So instead of rewriting related codes or forwarding/proxying the calls, this PR proposes to simply expose the $output by adding a getter to allow advanced users to implment those feature without rewriting the whole thing, and at the same time we don't have to worry about it breaks anything.

[jhdxr]

In case this PR get rejected or merged but reverted, or for people who stick with the old versions, here is a workaround, you can use Reflection to get any private attribute, including $output here:

        $output = $this->getOutput(); //this is OutputStyle
        $rOutput = new \ReflectionClass($output);
        $pOutput = $rOutput->getProperty('output');
        $pOutput->setAccessible(true);
        $rawOutput = $pOutput->getValue($output); //this is the underlying implmenetation of OutputInterface

Please note reflection is usually considered as slow, so this is just a workaround.