Contributor role enum value

[mike-podolskiy90]

According to the frictionless specs contributor's role is only recommended to be one of author, publisher, maintainer, wrangler, and contributor. Current implementation allows only values from the enum https://github.com/frictionlessdata/datapackage-java/blob/main/src/main/java/io/frictionlessdata/datapackage/Contributor.java#L98

There are plenty of cases where those five is not enough, so it should be String.

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@iSnow

[iSnow]

I see the problem, but this needs to be a bug report on the specs, we are following their recommendations: https://specs.frictionlessdata.io/data-package/#metadata