GitHub issue resolved: #5626

Briefly describe the changes proposed in this PR:

This PR adds support for custom aggregate functions with multiple arguments. It adds new factory and type for it (CustomAggregateNAryFunctionRegistry, AggregateNArgFunction) and extends SPARQL parser to support such functions. As a consequence, minor changes to AggregateFunctionCall were done too with addition of AbstractNAryAggregateOperator. I've tried to introduce no breaking changes and only extend the support as an opt-in feature.

PR Author Checklist (see the contributor guidelines for more details):

• my pull request is self-contained
• I've added tests for the changes I made
• I've applied code formatting (you can use mvn process-resources to format from the command line)
• I've squashed my commits where necessary
• every commit message starts with the issue number (GH-xxxx) followed by a meaningful description of the change