### 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
comparison = (BinaryComparison) filter.condition();
                }
                assertThat(comparison.left(), instanceOf(UnresolvedAttribute.class));
                assertThat(((UnresolvedAttribute) comparison.left()).name(), equalTo(expectedIdentifiers[j]));
                assertThat(comparison.right(), instanceOf(Literal.class));
                assertThat(((Literal) comparison.right()).value(), equalTo(123));
                assertThat(filter.child(), equalTo(PROCESSING_CMD_INPUT));
            }
        }
    }
    public void testBooleanLiteralCondition() {
        LogicalPlan where = processingCommand("where true");
        assertThat(where, instanceOf(Filter.class));
        Filter w = (Filter) where;
        assertThat(w.child(), equalTo(PROCESSING_CMD_INPUT));
        assertThat(w.condition(), equalTo(TRUE));
    }
    public void testBasicLimitCommand() {
        LogicalPlan plan = statement("from text | where true | limit 5");
        assertThat(plan, instanceOf(Limit.class));
        Limit limit = (Limit) plan;
        assertThat(limit.limit(), instanceOf(Literal.class));
        assertThat(((Literal) limit.limit()).value(), equalTo(5));
        assertThat(limit.children().size(), equalTo(1));
        assertThat(limit.children().get(0), instanceOf(Filter.class));
        assertThat(limit.children().get(0).children().size(), equalTo(1));
        assertThat(limit.children().get(0).children().get(0), instanceOf(EsqlUnresolvedRelation.class));
    }
    public void testLimitConstraints() {
        expectError("from text | limit -1", "extraneous input '-' expecting INTEGER_LITERAL");