### 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
}
    public void testDoNotExtractGroupingFields() {
        var plan = physicalPlan("""
            from test
            | stats x = sum(salary) by first_name
            """);
        var optimized = optimizedPlan(plan);
        var limit = as(optimized, LimitExec.class);
        var aggregate = as(limit.child(), AggregateExec.class);
        assertThat(aggregate.estimatedRowSize(), equalTo(Long.BYTES + KEYWORD_EST));
        assertThat(aggregate.groupings(), hasSize(1));
        var exchange = asRemoteExchange(aggregate.child());
        aggregate = as(exchange.child(), AggregateExec.class);
        assertThat(aggregate.estimatedRowSize(), equalTo(Long.BYTES + KEYWORD_EST));
        assertThat(aggregate.groupings(), hasSize(1));
        var extract = as(aggregate.child(), FieldExtractExec.class);
        assertThat(names(extract.attributesToExtract()), equalTo(List.of("salary")));
        var source = source(extract.child());
        
        
        assertThat(source.estimatedRowSize(), equalTo(Integer.BYTES * 2));
    }
    public void testExtractGroupingFieldsIfAggd() {
        var plan = physicalPlan("""
            from test
            | stats x = count(first_name) by first_name
            """);
        var optimized = optimizedPlan(plan);
        var limit = as(optimized, LimitExec.class);
        var aggregate = as(limit.child(), AggregateExec.class);