### 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
List.of(
                    new Alias(EMPTY, "a.b.c", integer(1)),
                    new Alias(EMPTY, "b", integer(2)),
                    new Alias(EMPTY, "@timestamp", new Literal(EMPTY, "2022-26-08T00:00:00", KEYWORD))
                )
            ),
            statement("row a.b.c = 1, `b` = 2, `@timestamp`=\"2022-26-08T00:00:00\"")
        );
    }
    public void testCompositeCommand() {
        assertEquals(
            new Filter(EMPTY, new Row(EMPTY, List.of(new Alias(EMPTY, "a", integer(1)))), TRUE),
            statement("row a = 1 | where true")
        );
    }
    public void testMultipleCompositeCommands() {
        assertEquals(
            new Filter(
                EMPTY,
                new Filter(EMPTY, new Filter(EMPTY, new Row(EMPTY, List.of(new Alias(EMPTY, "a", integer(1)))), TRUE), FALSE),
                TRUE
            ),
            statement("row a = 1 | where true | where false | where true")
        );
    }
    public void testEval() {
        assertEquals(
            new Eval(EMPTY, PROCESSING_CMD_INPUT, List.of(new Alias(EMPTY, "b", attribute("a")))),
            processingCommand("eval b = a")
        );