built_value codegen support

**Is your feature request related to a problem? Please describe.**
I want to start using built_value from now on on my projects, and a big thing that needs adressing is hive support, as i do not want to write TypeAdapter s manually.

**Describe the solution you'd like**
TypeAdapter codegen being built_value aware, being able to properly read and write like regular classes. Although, full proper support with HiveLists is a whole other beast because of the mutability of the structure

**Describe alternatives you've considered**
Im preparing the described feature, but i wanted to know if it is an welcome addition to hive in general or if i should keep my changes local.

**Version**
- Platform: iOS, Android, Mac, Windows, Linux, Web
- Hive version: any