### 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
public void testPushDownLimitPastGrok() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | grok first_name "%{WORD:y}"
            | limit 10""");
        var grok = as(plan, Grok.class);
        as(grok.child(), Limit.class);
    }
    public void testPushDownLimitPastProject() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | rename emp_no as a
            | keep a
            | limit 10""");
        var keep = as(plan, Project.class);
        as(keep.child(), Limit.class);
    }
    public void testDontPushDownLimitPastFilter() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | limit 100
            | where emp_no > 10
            | limit 10""");
        var limit = as(plan, Limit.class);
        var filter = as(limit.child(), Filter.class);
        as(filter.child(), Limit.class);
    }
    public void testEliminateHigherLimitDueToDescendantLimit() throws Exception {
        LogicalPlan plan = optimizedPlan("""
            from test