### Elasticsearch Version

main @ 1266718e1cdef07a8c7cd1a463707825fabf7277

### Installed Plugins

_No response_

### Java Version

openjdk 17.0.8 2023-07-18

### OS Version

Ubuntu 23.;04

### Problem Description

`DISSECT`'s option `append_separator` is not case insensitive in contrast to all other ESQL keywords.

### Steps to Reproduce

This query works:
```
POST _query?format=txt
{
  "query": """
ROW a = "john jacob jingleheimer schmidt"
| DISSECT a "%{+name} %{+name} %{+name} %{+name}" append_separator="---"
  """
}
```
The identical query with upper cased `APPEND_SEPARATOR` does not:
```
POST _query?format=txt
{
  "query": """
ROW a = "john jacob jingleheimer schmidt"
| DISSECT a "%{+name} %{+name} %{+name} %{+name}" APPEND_SEPARATOR="---"
  """
}
```
It fails with `Invalid option for dissect: [APPEND_SEPARATOR]`.

### Logs (if relevant)

_No response_
x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/parser/StatementParserTests.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.parser;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.test.ESTestCase;
import org.elasticsearch.xpack.esql.evaluator.predicate.operator.comparison.Equals;
import org.elasticsearch.xpack.esql.evaluator.predicate.operator.comparison.GreaterThan;
import org.elasticsearch.xpack.esql.evaluator.predicate.operator.comparison.GreaterThanOrEqual;
import org.elasticsearch.xpack.esql.evaluator.predicate.operator.comparison.LessThan;
import org.elasticsearch.xpack.esql.evaluator.predicate.operator.comparison.LessThanOrEqual;
import org.elasticsearch.xpack.esql.evaluator.predicate.operator.regex.RLike;
import org.elasticsearch.xpack.esql.evaluator.predicate.operator.regex.WildcardLike;
import org.elasticsearch.xpack.esql.expression.predicate.operator.arithmetic.Add;
import org.elasticsearch.xpack.esql.plan.logical.Dissect;
import org.elasticsearch.xpack.esql.plan.logical.Enrich;
import org.elasticsearch.xpack.esql.plan.logical.EsqlUnresolvedRelation;
import org.elasticsearch.xpack.esql.plan.logical.Eval;
import org.elasticsearch.xpack.esql.plan.logical.Explain;
import org.elasticsearch.xpack.esql.plan.logical.Grok;
import org.elasticsearch.xpack.esql.plan.logical.InlineStats;
import org.elasticsearch.xpack.esql.plan.logical.MvExpand;
import org.elasticsearch.xpack.esql.plan.logical.Row;
import org.elasticsearch.xpack.ql.expression.Alias;