GH-4489 User defined SPARQL queries in SHACL constraints and targets

[hmottestad]

GitHub issue resolved: #4489

Briefly describe the changes proposed in this PR:

• Support for SPARQL based constraints (from the SHACL spec)
• Support for SPARQL based targets (from the SHACL Advanced Features Working Group Note)

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[hmottestad]

I was considering if we should have a config option to enable the SHACL Advanced Features, but since it's in the same namespace I don't really want to complicate things.

[abrokenjester]

Haven't fully reviewed (it's a lot) but overall LGTM.

Slightly worried that we are duplicating existing SPARQL-related functionality inside the shaclsail module but I trust you won't let that get out of hand.