### 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
tuple("<=", LessThanOrEqual.class)
            
        )) {
            var plan = physicalPlan("from test [metadata _index] | where _index " + t.v1() + " \"test\"");
            var optimized = optimizedPlan(plan);
            var limit = as(optimized, LimitExec.class);
            var exchange = asRemoteExchange(limit.child());
            var project = as(exchange.child(), ProjectExec.class);
            var extract = as(project.child(), FieldExtractExec.class);
            limit = as(extract.child(), LimitExec.class);
            var filter = as(limit.child(), FilterExec.class);
            var comp = as(filter.condition(), t.v2());
            var metadataAttribute = as(comp.left(), MetadataAttribute.class);
            assertThat(metadataAttribute.name(), is("_index"));
            extract = as(filter.child(), FieldExtractExec.class);
            var source = source(extract.child());
        }
    }
    public void testDontPushDownMetadataVersionAndId() {
        for (var t : List.of(tuple("_version", "2"), tuple("_id", "\"2\""))) {
            var plan = physicalPlan("from test [metadata " + t.v1() + "] | where " + t.v1() + " == " + t.v2());
            var optimized = optimizedPlan(plan);
            var limit = as(optimized, LimitExec.class);
            var exchange = asRemoteExchange(limit.child());
            var project = as(exchange.child(), ProjectExec.class);
            var extract = as(project.child(), FieldExtractExec.class);
            limit = as(extract.child(), LimitExec.class);
            var filter = as(limit.child(), FilterExec.class);
            assertThat(filter.condition(), instanceOf(Equals.class));
            assertThat(((Equals) filter.condition()).left(), instanceOf(MetadataAttribute.class));