### 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
assertEquals(
            new Eval(
                EMPTY,
                PROCESSING_CMD_INPUT,
                List.of(new Alias(EMPTY, "b", attribute("a")), new Alias(EMPTY, "c", new Add(EMPTY, attribute("a"), integer(1))))
            ),
            processingCommand("eval b = a, c = a + 1")
        );
    }
    public void testEvalImplicitNames() {
        assertEquals(new Eval(EMPTY, PROCESSING_CMD_INPUT, List.of(new Alias(EMPTY, "a", attribute("a")))), processingCommand("eval a"));
        assertEquals(
            new Eval(
                EMPTY,
                PROCESSING_CMD_INPUT,
                List.of(
                    new Alias(
                        EMPTY,
                        "fn(a+1)",
                        new UnresolvedFunction(EMPTY, "fn", DEFAULT, List.of(new Add(EMPTY, attribute("a"), integer(1))))
                    )
                )
            ),
            processingCommand("eval fn(a + 1)")
        );
    }
    public void testStatsWithGroups() {
        assertEquals(
            new Aggregate(
                EMPTY,