### 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
String[] delims = new String[] { "", "\r", "\n", "\r\n" };
        Function<String, String> queryFun = d -> d + "from " + d + " foo " + d + "| eval " + d + " x = concat(bar, \"baz\")" + d;
        LogicalPlan reference = statement(queryFun.apply(delims[0]));
        for (int i = 1; i < delims.length; i++) {
            LogicalPlan candidate = statement(queryFun.apply(delims[i]));
            assertThat(candidate, equalTo(reference));
        }
    }
    public void testSuggestAvailableSourceCommandsOnParsingError() {
        for (Tuple<String, String> queryWithUnexpectedCmd : List.of(
            Tuple.tuple("frm foo", "frm"),
            Tuple.tuple("expln[from bar]", "expln"),
            Tuple.tuple("not-a-thing logs", "not-a-thing"),
            Tuple.tuple("high5 a", "high5"),
            Tuple.tuple("a+b = c", "a+b"),
            Tuple.tuple("a//hi", "a"),
            Tuple.tuple("a/*hi*/", "a"),
            Tuple.tuple("explain [ frm a ]", "frm")
        )) {
            ParsingException pe = expectThrows(ParsingException.class, () -> statement(queryWithUnexpectedCmd.v1()));
            assertThat(
                pe.getMessage(),
                allOf(
                    containsString("mismatched input '" + queryWithUnexpectedCmd.v2() + "'"),
                    containsString("'explain'"),
                    containsString("'from'"),
                    containsString("'row'")
                )
            );
        }