Add GetOrCreateLink extension method that returns not an address but all link's contents

This method can be a replacement for `links.GetLink(links.GetOrCreate(...))` to make code even shorter.

I got this idea then was answering https://stackoverflow.com/questions/67210209/how-i-can-get-each-source-and-target-from-doublet-of-doublets question.

Start by reading the existing links.GetLink and links.GetOrCreate entry points to understand how they are composed. Add the proposed GetOrCreateLink extension method so it returns the link's contents rather than an address, and confirm it can replace the composition described in the issue.

Written by the indexing model from the issue text.