[RFE] Expose optional attribute toggle in schema config

[afflom]

What is the problem you're trying to solve
All attributes within the schema are currently required to be set on all collection items

Describe the solution you'd like
Expose optional attribute toggle in schema config that allows for attributes to be marked as optional.

[afflom]

On the topic of required vs optional: There needs to be two categories of "required":

1. Required keys; where a specific key is required to be annotated to all objects within a collection (with the exception of the schema link)

2. Required values, where a value must exist within a collection. A good example of this is index.html. index.html needs to be annotated as the entrypoint to the content of the collection. The requirement here should be that only one object within the collection can be the entrypoint and that entrypoint must exist within the collection.