Unreal: Added settings for rendering

[simonebarbieri]

Brief description

Added settings for rendering in Unreal Engine.

Description

Two settings has been added:

• Pre roll frames, to set how many frames are used to load the scene before starting the actual rendering.
• Configuration path, to allow to save a preset of settings from Unreal, and use it for rendering.

Testing notes:

A layout will need to be loaded from Maya or Blender to be able to render the shot.
To render:

1. Create a render instance for a sequence.
2. Select the render instance and select Render from the OpenPype menu.

You can set the settings from the Studio Settings/Project/Unreal Engine.
For the Pre-roll frames, you can just set any number of frames.
For the Render Config Path, it should work both with an empty setting (and in that case it will use some default settings), or by setting a path with the Unreal format (e.g. /Game/Settings/Config.Config).

[antirotor]

Since we are touching settings, shouldn't we add at least option to change file format? If I am not mistaken, now it renders to pngs hardcoded, but it should probably support more formats? Or is it a topic for another PR (along with the resolution that is now hardcoded?)