fix: global headers are only required if the underlying type is required

[armandobelardo]

This PR makes it so that:

1. if a header is required on some endpoints and optional on others then it cannot be a global header
2. if a header is optional on every endpoint, it's an optional global header (previously it would be considered required, regardless of the underlying type)
3. a global header is only required if it's required on every endpoint