### 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
PROCESSING_CMD_INPUT,
                List.of(attribute("c"), attribute("d.e")),
                List.of(
                    new Alias(EMPTY, "b", new UnresolvedFunction(EMPTY, "min", DEFAULT, List.of(attribute("a")))),
                    attribute("c"),
                    attribute("d.e")
                )
            ),
            processingCommand("stats b = min(a) by c, d.e")
        );
    }
    public void testStatsWithoutGroups() {
        assertEquals(
            new Aggregate(
                EMPTY,
                PROCESSING_CMD_INPUT,
                List.of(),
                List.of(
                    new Alias(EMPTY, "min(a)", new UnresolvedFunction(EMPTY, "min", DEFAULT, List.of(attribute("a")))),
                    new Alias(EMPTY, "c", integer(1))
                )
            ),
            processingCommand("stats min(a), c = 1")
        );
    }
    public void testStatsWithoutAggs() throws Exception {
        assertEquals(
            new Aggregate(EMPTY, PROCESSING_CMD_INPUT, List.of(attribute("a")), List.of(attribute("a"))),
            processingCommand("stats by a")
        );