Move IDataMigrationManager.UpdateAsync() to an extension method

[hishamco]

While i'm working on data docs, I notice that IDataMigrationManager has two versions of UpdateAsync() one accept feature and the other one accepts a list of features

IMHO the UpdateAsync(IEumerable<string> features) could be an extension methods instead

/cc @jtkech

[Piedone]

I'd rather do it the other way around: UpdateAsync(IEnumerable<string> featureIds) can (and actually does) optimize updating multiple features and do it better than just call UpdateAsync(string featureId) in a loop.

[hishamco]

Even though moving it into an extension method makes sense, then we can find a way to optimize it