Add weak generics for array type objects

[LauJosefsen]

Summary

Add weak default generics of the kind <array-key, mixed> for default casts that returns those generics, i.e. json, array, arrayable and collection casts.

Solves #1614

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Misc. change (internal, infrastructure, maintenance, etc.)

Checklist

• Existing tests have been adapted and/or new tests have been added
• Update the README.md
• Code style has been fixed via composer fix-style

[chescos]

Could we get this merged?