### 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
);
    }
    public void testSubquerySpacing() {
        assertEquals(statement("explain [ explain [ from a ] | where b == 1 ]"), statement("explain[explain[from a]|where b==1]"));
    }
    public void testBlockComments() {
        String query = " explain [ from foo ] | limit 10 ";
        LogicalPlan expected = statement(query);
        int wsIndex = query.indexOf(' ');
        do {
            String queryWithComment = query.substring(0, wsIndex)
                + "/*explain [ \nfrom bar ] | where a > b*/"
                + query.substring(wsIndex + 1);
            assertEquals(expected, statement(queryWithComment));
            wsIndex = query.indexOf(' ', wsIndex + 1);
        } while (wsIndex >= 0);
    }
    public void testSingleLineComments() {
        String query = " explain [ from foo ] | limit 10 ";
        LogicalPlan expected = statement(query);
        int wsIndex = query.indexOf(' ');
        do {
            String queryWithComment = query.substring(0, wsIndex)
                + "//explain [ from bar ] | where a > b \n"
                + query.substring(wsIndex + 1);
            assertEquals(expected, statement(queryWithComment));
            wsIndex = query.indexOf(' ', wsIndex + 1);
        } while (wsIndex >= 0);
    }
    public void testNewLines() {