### Description

Evals that introduce aliased can be simplified by extracting them into a project (and thus signaling there's no underlying processing).

The following eval:
`eval x = a + 1, y = x, z = y + 1, y = z, w = y + 1`
can be converted into:
`eval x = a + 1, z = a + 1 + 1, w = a + 1 + 1 | project x, z, z as y, w`

This helps fuse the project with other projections/aggs downstream.
x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizer.java
ConvertStringToByteRef() {
            super(TransformDirection.UP);
        }
        @Override
        protected Expression rule(Literal lit) {
            if (lit.value() == null) {
                return lit;
            }
            if (lit.value() instanceof String s) {
                return Literal.of(lit, new BytesRef(s));
            }
            if (lit.value() instanceof List<?> l) {
                if (l.isEmpty() || false == l.get(0) instanceof String) {
                    return lit;
                }
                return Literal.of(lit, l.stream().map(v -> new BytesRef((String) v)).toList());
            }
            return lit;
        }
    }
    static class CombineProjections extends OptimizerRules.OptimizerRule<UnaryPlan> {