Support "Mirrored Sprite Frames"

[sunkin351]

Similar to #5878 but for SpriteFrames.

I'm porting an existing project from an in-house engine to Godot. In order to use my existing assets (specifically convert them to godot SpriteFrames), I need the ability to flip/mirror individual frames on the vertical axis (the horizontal axis would be cool too) as part of the animation. And while I might be able to do this myself manually, that'd be a lot of effort that needs to be replicated across projects, on top of the fact that I wouldn't be able to use godot's built-in asset format.

Points of interest:

• Editor Support
• SpriteFrames API Support

This will likely require a change to the file format, though I imagine this could be done in a backwards-compatible way.
(Disclaimer: I am not speaking with any knowledge of what the file format looks like)