feat: add custom DTO factory support for mixed model types

[abdulhaq-e]

Description

This allows for nesting mixed models, e.g. a SQLAlchemy model in a dataclass or any other mixture of models.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.34%. Comparing base (7f949b3) to head (78724c9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4123   +/-   ##
=======================================
  Coverage   98.34%   98.34%           
=======================================
  Files         348      348           
  Lines       15841    15848    +7     
  Branches     1749     1749           
=======================================
+ Hits        15579    15586    +7     
  Misses        125      125           
  Partials      137      137           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/4123

[provinzkraut]

The feature itself looks good, the implementation approach isn't ideal imo. This generates nested DTOs, whereas I think it should instead generate one DTO that's nested. Especially for the codegen backend, this should be much more efficient.