feat: custom converter types

[antazoey]

What I did

Allow you to implement mixin hooks for a custom class to be convertible to different types.
I have a weird wrapper type that can't easily inherit from ContractInstance and I am constantly accidentally passing it to transactions but i of course get conversion errors. With this, I can implement these hooks and my class will work as arguments much like a regular ContractInstance does.

fixes: #2624

How I did it

How to verify it

Checklist

• All changes are completed
• Change is covered in tests
• Documentation is complete

[fubuloubu]

I like this! Perhaps we can do something in v0.9/v1.x series to make this more generalizable/performant, but for now this works well!