### 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/PhysicalPlanOptimizerTests.java
| stats x = sum(c)
            """);
        var optimized = optimizedPlan(plan);
        var limit = as(optimized, LimitExec.class);
        var aggregate = as(limit.child(), AggregateExec.class);
        assertThat(aggregate.estimatedRowSize(), equalTo(Long.BYTES));
        var exchange = asRemoteExchange(aggregate.child());
        aggregate = as(exchange.child(), AggregateExec.class);
        assertThat(aggregate.estimatedRowSize(), equalTo(Long.BYTES));
        var eval = as(aggregate.child(), EvalExec.class);
        var extract = as(eval.child(), FieldExtractExec.class);
        assertThat(names(extract.attributesToExtract()), contains("salary"));
        var filter = as(extract.child(), FilterExec.class);
        extract = as(filter.child(), FieldExtractExec.class);
        assertThat(names(extract.attributesToExtract()), contains("emp_no"));
        var query = source(extract.child());
        assertThat(query.estimatedRowSize(), equalTo(Integer.BYTES * 4 ));
    }
    public void testTripleExtractorPerField() {
        var plan = physicalPlan("""
            from test
            | where round(emp_no) > 10
            | eval c = first_name
            | stats x = sum(salary)
            """);
        var optimized = optimizedPlan(plan);
        var limit = as(optimized, LimitExec.class);
        var aggregate = as(limit.child(), AggregateExec.class);
        var exchange = asRemoteExchange(aggregate.child());
        aggregate = as(exchange.child(), AggregateExec.class);