### 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 testPushDownDisjunctionAndConjunction() {
        var plan = physicalPlan("""
            from test
            | where first_name == "Bezalel" or first_name == "Suzette"
            | where salary > 50000
            """);
        var optimized = optimizedPlan(plan);
        var topLimit = as(optimized, LimitExec.class);
        var exchange = asRemoteExchange(topLimit.child());
        var project = as(exchange.child(), ProjectExec.class);
        var extractRest = as(project.child(), FieldExtractExec.class);
        var source = source(extractRest.child());
        assertThat(source.estimatedRowSize(), equalTo(allFieldRowSize + Integer.BYTES));
        BoolQueryBuilder query = as(source.query(), BoolQueryBuilder.class);
        assertThat(query.must(), hasSize(2));
        var tq = as(sv(query.must().get(0), "first_name"), TermsQueryBuilder.class);
        assertThat(tq.fieldName(), is("first_name"));
        assertThat(tq.values(), is(List.of("Bezalel", "Suzette")));
        var rqb = as(sv(query.must().get(1), "salary"), RangeQueryBuilder.class);
        assertThat(rqb.fieldName(), is("salary"));
        assertThat(rqb.from(), is(50_000));
        assertThat(rqb.includeLower(), is(false));
        assertThat(rqb.to(), nullValue());
    }
    public void testPushDownIn() {
        var plan = physicalPlan("""
            from test
            | where emp_no in (10020, 10030 + 10)
            """);
        var optimized = optimizedPlan(plan);