### 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/test/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizerTests.java
public void testCombineOrderByThroughProjectAndEval() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no
            | rename emp_no as en
            | keep salary, en
            | eval e = en * 2
            | sort salary""");
        var keep = as(plan, Project.class);
        var topN = as(keep.child(), TopN.class);
        assertThat(orderNames(topN), contains("salary", "emp_no"));
        var eval = as(topN.child(), Eval.class);
        assertThat(Expressions.names(eval.fields()), contains("e"));
        as(eval.child(), EsRelation.class);
    }
    public void testCombineOrderByThroughProjectWithAlias() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no
            | rename salary as l
            | keep l, emp_no
            | sort l""");
        var keep = as(plan, Project.class);
        var topN = as(keep.child(), TopN.class);
        assertThat(orderNames(topN), contains("salary", "emp_no"));
        as(topN.child(), EsRelation.class);
    }
    public void testCombineOrderByThroughFilter() {
        LogicalPlan plan = optimizedPlan("""
            from test