Add a FunctionContributor contract

[maesenka]

Analogously to TypeContributor, it is useful for extension modules (e.g. Spatial) to be able to provide additional functions to the QueryEngine.

The contract would be:

public interface FunctionContributor {
   void contributeFunctions(FunctionContributions functionContributions, ServiceRegistry serviceRegistry);
}

public interface FunctionContributions {
        void contributeFunction(String registrationKey, SqmFunctionDescriptor function);
}

See issue HHH-14754

[maesenka]

I have submitted a PR for this #4109