### 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/LogicalPlanOptimizerTests.java
| rename a as x
            | keep x
            | enrich languages_idx on x
            """);
        var keep = as(plan, Project.class);
        as(keep.child(), Enrich.class);
    }
    public void testTopNEnrich() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | rename languages as x
            | eval x = to_string(x)
            | keep x
            | enrich languages_idx on x
            | sort language_name
            """);
        var keep = as(plan, Project.class);
        var topN = as(keep.child(), TopN.class);
        as(topN.child(), Enrich.class);
    }
    public void testEnrichNotNullFilter() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | eval x = to_string(languages)
            | enrich languages_idx on x
            | where language_name is not null
            | limit 10
            """);
        var limit = as(plan, Limit.class);
        var filter = as(limit.child(), Filter.class);