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, 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.

[grape-bot]

	1 Warning
⚠️	Unless you're refactoring existing code or improving documentation, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#81](https://github.com/ruby-grape/grape-swagger-entity/pull/81): Make entity fields required by default - [@olivier-thatch](https://github.com/olivier-thatch).

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[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.