[ULX-1596] feat: add custom prompts partials

[joel-hamilton]

🔧 Changes

• This PR adds support for importing/exporting custom prompts partials (feature docs). This feature allows customers to add HTML/Liquid to various extensibility points in the Login/Signup prompts.
• We add a new prompts/partials.json config, and a partials directory for the actual partials (see example below)

Note:

• we should consider implementing a flat structure and just naming our templates {prompt}/{screen}/{insertion_point}.liquid
• otherwise, we'll need to add another layer of directory nesting to capture the screen (the one below is missing a subdirectory for login and for signup
  

📚 References

ULX-1596

🔬 Testing

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[ifyoumakeit]

Hi! Is there any update on when this will be part of the CLI?