### 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
}
    private static Literal literalUnsignedLongs(String... ulongs) {
        return new Literal(EMPTY, Arrays.stream(ulongs).map(s -> asLongUnsigned(new BigInteger(s))).toList(), DataTypes.UNSIGNED_LONG);
    }
    private static Literal literalBoolean(boolean b) {
        return new Literal(EMPTY, b, DataTypes.BOOLEAN);
    }
    private static Literal literalBooleans(boolean... booleans) {
        List<Boolean> v = new ArrayList<>(booleans.length);
        for (boolean b : booleans) {
            v.add(b);
        }
        return new Literal(EMPTY, v, DataTypes.BOOLEAN);
    }
    private static Literal literalString(String s) {
        return new Literal(EMPTY, s, DataTypes.KEYWORD);
    }
    private static Literal literalStrings(String... strings) {
        return new Literal(EMPTY, Arrays.asList(strings), DataTypes.KEYWORD);
    }
    private void expectError(String query, String errorMessage) {
        ParsingException e = expectThrows(ParsingException.class, "Expected syntax error for " + query, () -> statement(query));
        assertThat(e.getMessage(), containsString(errorMessage));
    }
    private void expectError(String query, List<TypedParamValue> params, String errorMessage) {
        ParsingException e = expectThrows(ParsingException.class, "Expected syntax error for " + query, () -> statement(query, params));
        assertThat(e.getMessage(), containsString(errorMessage));
    }
}