### 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
}
    public void testBasicSortCommand() {
        LogicalPlan plan = statement("from text | where true | sort a+b asc nulls first, x desc nulls last | sort y asc | sort z desc");
        assertThat(plan, instanceOf(OrderBy.class));
        OrderBy orderBy = (OrderBy) plan;
        assertThat(orderBy.order().size(), equalTo(1));
        Order order = orderBy.order().get(0);
        assertThat(order.direction(), equalTo(Order.OrderDirection.DESC));
        assertThat(order.nullsPosition(), equalTo(Order.NullsPosition.FIRST));
        assertThat(order.child(), instanceOf(UnresolvedAttribute.class));
        assertThat(((UnresolvedAttribute) order.child()).name(), equalTo("z"));
        assertThat(orderBy.children().size(), equalTo(1));
        assertThat(orderBy.children().get(0), instanceOf(OrderBy.class));
        orderBy = (OrderBy) orderBy.children().get(0);
        assertThat(orderBy.order().size(), equalTo(1));
        order = orderBy.order().get(0);
        assertThat(order.direction(), equalTo(Order.OrderDirection.ASC));
        assertThat(order.nullsPosition(), equalTo(Order.NullsPosition.LAST));
        assertThat(order.child(), instanceOf(UnresolvedAttribute.class));
        assertThat(((UnresolvedAttribute) order.child()).name(), equalTo("y"));
        assertThat(orderBy.children().size(), equalTo(1));
        assertThat(orderBy.children().get(0), instanceOf(OrderBy.class));
        orderBy = (OrderBy) orderBy.children().get(0);
        assertThat(orderBy.order().size(), equalTo(2));
        order = orderBy.order().get(0);
        assertThat(order.direction(), equalTo(Order.OrderDirection.ASC));
        assertThat(order.nullsPosition(), equalTo(Order.NullsPosition.FIRST));
        assertThat(order.child(), instanceOf(Add.class));
        Add add = (Add) order.child();
        assertThat(add.left(), instanceOf(UnresolvedAttribute.class));