feat(core): add SubstraitBuilder.fieldReference(s) methods for joins

[nielspardon]

This PR adds two convenience methods to SubstraitBuilder for creating field references from a JoinInput. Simplifies patterns like these:

Rel left;
Rel right;
sb.innerJoin(inputs -> sb.equal(sb.fieldReference(List.of(inputs.left, inputs.right), 0), sb.fieldReference(List.of(inputs.left, inputs.right), 10)), left, right);

// or 

Rel left;
Rel right;
List<Rel> inputRels = List.of(left, right);
sb.innerJoin(inputs -> sb.equal(sb.fieldReference(inputRels, 0), sb.fieldReference(inputRels, 10)), left, right);

to simply:

Rel left;
Rel right;
sb.innerJoin(inputs -> sb.equal(sb.fieldReference(inputs, 0), sb.fieldReference(inputs, 10)), left, right);

[nielspardon]

public FieldReference fieldReference(List<Rel> inputs, int index) mainly remains for backwards compatibility and for extension multi relations that have expressions but is otherwise no longer used in this repo