Allow enums in `@visibility` decorator

[timotheeguerin]

Playground Link

Things to think about:

• what to we do with existing string value(deprecate?)
• change the api to use union? @visibility(HttpVis.read | HttpVis.write)
• multiple @visibility does it append or override: We definitly need some appending functionality so you can provide visibiity foir different protocol but not being able to override might be limiting.