GH-4997: Enable SparqlBuilder to create VALUES clauses

[fkleedorfer]

GitHub issue resolved: #4997

Briefly describe the changes proposed in this PR:

Adds a builder for VALUES clauses and integrates them with GraphPattern and Select

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[fkleedorfer]

Happy to rebase that onto develop and change the target branch to develop, if needed.

[hmottestad]

Happy to rebase that onto develop and change the target branch to develop, if needed.

Yeah, this needs to target the develop branch.

Btw. Have you considered supporting VALUES inside UNION clauses? It's a common scenario for getting better performance since the scoping of the UNION means that any VALUES clauses outside the UNION clause will be evaluated after the UNION clauses. Duplicating the VALUES clause inside each branch of the UNION clause fixes the scoping issue so the VALUES clause can be used when executing the joins inside each UNION clause.

[fkleedorfer]

You can add it to any GraphPattern, so you can also add it to a Union pattern

[fkleedorfer]

... But we might be able to make that more explicit if you like
EDIT: looked into that - there does not seem to be a good way to make it more explicit. Just add the values clause to your graph pattern.

It is generally a weakness of the sparqlbuilder package that you have to know how to produce the bits you need. The API has little 'fluency'. This here is a case in point.