Expose a method to override rendering of the final generated image

[thomas-stockx]

We prefer the golden_toolkit methodology of setting up golden tests above other packages currently available.
However, probably the number one issue at this point is consistent UI tests across platforms. Even with the Ahem font, these squares do not render consistently accross platforms, due to issues with font smoothing, text scaling differences, etc.
Other packages (such as alchemist) have workarounds for this, like replacing all rendered paragraphs with rendered rectangles instead.

To allow us the best of both worlds, I exposed a new function in the configuration for golden_toolkit which allows providing a method which will run right before the golden tests are generated. This allows developers to (for example) hook-in the rectangle rendering for fonts of the alchemist package (https://github.com/Betterment/alchemist/blob/main/lib/src/blocked_text_image.dart) which will generate consistent golden tests across all platforms, or do more advanced stuff.

[mZadorskii]

@thomas-stockx Thanks for contribution!
Can you add a test with golden that is leveraging new functionality?