Add Datasets methods without type.

This doesn't need to hold up 0.15.0, but if we can get it in, I'd like to.

When updating the examples after CDK-511, it was apparent that some calls to the Datasets API are a little awkward when the user doesn't care about the record type or dataset. For example:

// caller doesn't use the created dataset,
// but type for the instance is required
Datasets.create("uri", descriptor, Object.class);

I think we should introduce variants that have no type argument and return Dataset<GenericData.Record>.