Add loadAppFonts

[robiness]

This introduces loadAppFonts functionality to Spot, primarily for use when takeScreenshot involves rendering any Text.

• [ ] Before merging, I want to explore utilizing Flutter SDK’s Roboto font instead of bundling it with Spot.
• [ ] Additionally, I plan to ensure third-party package fonts function correctly and verify that font overrides as in _overridableFonts is whats expected.
• [ ] We should try to load the fonts from the system if available
• [ ] We may also load roboto from google fonts
• [ ] Use a better golden page
• [ ] we may also add a loadBlockFont which overrides the used block font