### 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
import static org.hamcrest.Matchers.allOf;
import static org.hamcrest.Matchers.containsString;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.instanceOf;
import static org.hamcrest.Matchers.is;
public class StatementParserTests extends ESTestCase {
    private static String FROM = "from test";
    EsqlParser parser = new EsqlParser();
    public void testRowCommand() {
        assertEquals(
            new Row(EMPTY, List.of(new Alias(EMPTY, "a", integer(1)), new Alias(EMPTY, "b", integer(2)))),
            statement("row a = 1, b = 2")
        );
    }
    public void testRowCommandImplicitFieldName() {
        assertEquals(
            new Row(
                EMPTY,
                List.of(new Alias(EMPTY, "1", integer(1)), new Alias(EMPTY, "2", integer(2)), new Alias(EMPTY, "c", integer(3)))
            ),
            statement("row 1, 2, c = 3")
        );
    }
    public void testRowCommandLong() {
        assertEquals(new Row(EMPTY, List.of(new Alias(EMPTY, "c", literalLong(2147483648L)))), statement("row c = 2147483648"));
    }
    public void testRowCommandHugeInt() {
        assertEquals(
            new Row(EMPTY, List.of(new Alias(EMPTY, "c", literalUnsignedLong("9223372036854775808")))),
            statement("row c = 9223372036854775808")