Make entity fields required by default

[olivier-thatch]

This PR changes how grape-swagger-entity determines if an entity field is required or not:

1. if documentation: { required: true/false } is set, use that
2. otherwise, if if or unless is not null or if expose_nil is false, assume the parameter is not required
3. otherwise, assume the parameter is required

This is consistent with how grape-entity renders fields: fields are always exposed unless if or unless is provided (cf. https://github.com/ruby-grape/grape-entity?tab=readme-ov-file#conditional-exposure).

Closes #80.

This is probably a breaking change, so I understand if you don't want to merge this as is, but it would be nice to be able to opt into this behavior.

[numbata]

Thanks a lot for this contribution, @olivier-thatch! The changes look generally good, and I appreciate that you not only highlighted this problem, but also suggested a solution. I do have some thoughts on a potential simplification in one of the methods, which I've detailed in a comment below.

[olivier-thatch]

I've updated the PR to account for another case of conditional exposure: expose_nil: false (cf. docs).