### 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
assertWarnings("PROJECT command is no longer supported, please use KEEP instead");
    }
    public void testInputParams() {
        LogicalPlan stm = statement(
            "row x = ?, y = ?, a = ?, b = ?, c = ?",
            List.of(
                new TypedParamValue("integer", 1),
                new TypedParamValue("keyword", "2"),
                new TypedParamValue("date_period", "2 days"),
                new TypedParamValue("time_duration", "4 hours"),
                new TypedParamValue("version", "1.2.3")
            )
        );
        assertThat(stm, instanceOf(Row.class));
        Row row = (Row) stm;
        assertThat(row.fields().size(), is(5));
        NamedExpression field = row.fields().get(0);
        assertThat(field.name(), is("x"));
        assertThat(field, instanceOf(Alias.class));
        Alias alias = (Alias) field;
        assertThat(alias.child().fold(), is(1));
        field = row.fields().get(1);
        assertThat(field.name(), is("y"));
        assertThat(field, instanceOf(Alias.class));
        alias = (Alias) field;
        assertThat(alias.child().fold(), is("2"));
        field = row.fields().get(2);
        assertThat(field.name(), is("a"));
        assertThat(field, instanceOf(Alias.class));
        alias = (Alias) field;