Feat/support examples property

[Flix6x]

I realize you did some work yourself on supporting the examples property, but I'm not sure if that is still ongoing. The examples branch hasn't been updated in 3 years.

Related discussion on supporting OpenAPI 3.1: #579

The OpenAPI upgrading docs suggest moving from example to examples, but as of now, both seem to be supported, so I haven't removed the singular example from the list of valid properties.

[lafrech]

Hi. Thanks for contributing.
In flask-smorest, I use examples extensively (see for instance marshmallow-code/flask-smorest#68) without the need for this change here. But there I do it at schema level while this change acts at field level, which I guess is fine, too, so I guess you're (I was) right about it.
Perhaps the unit tests would clarify the use case. Do you think you could add one?

[Flix6x]

Thanks, indeed my use case is field examples rather than schema examples. Something like this? I noticed two things:

1. The test would fail on dev, as expected.
2. The test succeeds on feat/support-examples-property, which I feel is somewhat unexpected, given that the fixture checks version 2.0 and 3.0.0. At first I thought the field examples were introduced in v3.1, but I think 3.0 already had them. 2.0 not, though.