Adds support for SPARQL CDTs (lists and maps as literals)

[hartig]

This closes #2518

The lack of built-in support for generic types of composite values such as lists and maps has been a long-standing issue for RDF and SPARQL. Together with a few other colleagues at the Amazon Neptune team we have developed an approach to represent lists and maps as literals in RDF data, and to extend SPARQL with features related to such literals. These extensions of SPARQL include:

• an aggregation function to produce these composite values (FOLD),
• functions to operate on these composite values in expressions, and
• a new operator (UNFOLD) to unfold such composite values into their individual components.

We have created a complete formal specification of the approach and a comprehensive test suite for implementers, which can be found in our Github repo: https://github.com/awslabs/SPARQL-CDTs

... and I have implemented a complete integration of this approach into Jena, as can be found in this PR. We would like to contribute this implementation to Jena if you are interested, and I will be more than happy to assist you with getting the PR ready to be merged.

Perhaps before you dive into the aforementioned specification, you may take a look at our short paper, in which we provide a slightly more extensive motivation for this work and a (very!) brief summary of the approach. After that, Section 2 of the specification provides a more detailed informal description of the different parts of the approach.

I am happy to answer any questions that you may have, both about the approach in general and about the implementation in this PR. Also, if you have issues with some parts of the specification, feel free to create an issue in the aforementioned Github repo. (And in case you are wondering, yes we are planning to file the approach as a SPARQL Enhancement Proposal (SEP) for the SPARQL-dev Community Group).

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• [ ] Tests are included.
• [ ] Documentation change and updates are provided for the Apache Jena website
• [ ] Commits have been squashed to remove intermediate development commit messages.
• [ ] Key commit messages start with the issue number (GH-xxxx)

By submitting this pull request, I acknowledge that I am making a contribution to the Apache Software Foundation under the terms and conditions of the Contributor's Agreement.

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See the Apache Jena "Contributing" guide.