### Description

As we have more rules doing analysis across plans, a number of methods are useful to add to the `QueryPlan` class itself, mainly around expressions:

```
- expressions() - return the expressions declared by this plan
- references() - return the references from the expressions above
```
x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizer.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.esql.optimizer;
import org.apache.lucene.util.BytesRef;
import org.elasticsearch.compute.data.BlockUtils;
import org.elasticsearch.xpack.esql.EsqlIllegalArgumentException;
import org.elasticsearch.xpack.esql.expression.SurrogateExpression;
import org.elasticsearch.xpack.esql.expression.function.aggregate.Count;
import org.elasticsearch.xpack.esql.expression.predicate.operator.comparison.In;
import org.elasticsearch.xpack.esql.plan.logical.Enrich;
import org.elasticsearch.xpack.esql.plan.logical.Eval;
import org.elasticsearch.xpack.esql.plan.logical.RegexExtract;
import org.elasticsearch.xpack.esql.plan.logical.TopN;
import org.elasticsearch.xpack.esql.plan.logical.local.EsqlProject;
import org.elasticsearch.xpack.esql.plan.logical.local.LocalRelation;
import org.elasticsearch.xpack.esql.plan.logical.local.LocalSupplier;
import org.elasticsearch.xpack.esql.type.EsqlDataTypes;
import org.elasticsearch.xpack.ql.expression.Alias;
import org.elasticsearch.xpack.ql.expression.Attribute;
import org.elasticsearch.xpack.ql.expression.AttributeMap;
import org.elasticsearch.xpack.ql.expression.AttributeSet;
import org.elasticsearch.xpack.ql.expression.Expression;
import org.elasticsearch.xpack.ql.expression.ExpressionSet;
import org.elasticsearch.xpack.ql.expression.Expressions;