### 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
var optimized = optimizedPlan(physicalPlan("""
            from test
            | limit 10
            """));
        var topLimit = as(optimized, LimitExec.class);
        var exchange = asRemoteExchange(topLimit.child());
        var project = as(exchange.child(), ProjectExec.class);
        var fieldExtract = as(project.child(), FieldExtractExec.class);
        var source = source(fieldExtract.child());
        assertThat(source.estimatedRowSize(), equalTo(allFieldRowSize + Integer.BYTES));
        assertThat(source.limit().fold(), is(10));
    }
    /**
     * TopNExec[[Order[nullsum{r}#3,ASC,LAST]],1[INTEGER]]
     * \_ExchangeExec[]
     *   \_ProjectExec[[_meta_field{f}#11, emp_no{f}#5, first_name{f}#6, !gender, languages{f}#8, last_name{f}#9, salary{f}#10, nulls
     * um{r}#3]]
     *     \_FieldExtractExec[_meta_field{f}#11, emp_no{f}#5, first_name{f}#6, !g..]
     *       \_TopNExec[[Order[nullsum{r}#3,ASC,LAST]],1[INTEGER]]
     *         \_EvalExec[[null[INTEGER] AS nullsum]]
     *           \_EsQueryExec[test], query[][_doc{f}#12], limit[], sort[]
     */
    public void testExtractorForEvalWithoutProject() throws Exception {
        var optimized = optimizedPlan(physicalPlan("""
            from test
            | eval nullsum = emp_no + null
            | sort nullsum
            | limit 1
            """));
        var topN = as(optimized, TopNExec.class);