### 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
assertThat(source.estimatedRowSize(), equalTo(Integer.BYTES * 5 + KEYWORD_EST));
    }
    /**
     * Expected
     *
     * EvalExec[[emp_no{f}#248 * 10[INTEGER] AS emp_no_10]]
     * \_LimitExec[10[INTEGER]]
     *   \_ExchangeExec[]
     *     \_ProjectExec[[_meta_field{f}#247, emp_no{f}#248, first_name{f}#249, languages{f}#250, last_name{f}#251, salary{f}#252]]
     *       \_FieldExtractExec[_meta_field{f}#247, emp_no{f}#248, first_name{f}#24..]
     *         \_EsQueryExec[test], query[][_doc{f}#253], limit[10], sort[]
     */
    public void testPushLimitToSource() {
        var optimized = optimizedPlan(physicalPlan("""
            from test
            | eval emp_no_10 = emp_no * 10
            | limit 10
            """));
        var eval = as(optimized, EvalExec.class);
        var topLimit = as(eval.child(), LimitExec.class);
        var exchange = asRemoteExchange(topLimit.child());
        var project = as(exchange.child(), ProjectExec.class);
        var extract = as(project.child(), FieldExtractExec.class);
        var leaves = extract.collectLeaves();
        assertEquals(1, leaves.size());
        var source = as(leaves.get(0), EsQueryExec.class);
        assertThat(source.limit().fold(), is(10));
        
        assertThat(source.estimatedRowSize(), equalTo(allFieldRowSize + Integer.BYTES * 2));