### 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
assertIdentifierAsIndexPattern("foo,test,xyz", "from foo,   test,xyz");
        assertIdentifierAsIndexPattern(
            "<logstash-{now/M{yyyy.MM}}>,<logstash-{now/d{yyyy.MM.dd|+12:00}}>",
            "from <logstash-{now/M{yyyy.MM}}>, `<logstash-{now/d{yyyy.MM.dd|+12:00}}>`"
        );
    }
    public void testIdentifierAsFieldName() {
        String[] operators = new String[] { "==", "!=", ">", "<", ">=", "<=" };
        Class<?>[] expectedOperators = new Class<?>[] {
            Equals.class,
            Not.class,
            GreaterThan.class,
            LessThan.class,
            GreaterThanOrEqual.class,
            LessThanOrEqual.class };
        String[] identifiers = new String[] { "abc", "`abc`", "ab_c", "a.b.c", "@a", "a.@b", "`a@b.c`" };
        String[] expectedIdentifiers = new String[] { "abc", "abc", "ab_c", "a.b.c", "@a", "a.@b", "a@b.c" };
        LogicalPlan where;
        for (int i = 0; i < operators.length; i++) {
            for (int j = 0; j < identifiers.length; j++) {
                where = processingCommand("where " + identifiers[j] + operators[i] + "123");
                assertThat(where, instanceOf(Filter.class));
                Filter filter = (Filter) where;
                assertThat(filter.children().size(), equalTo(1));
                assertThat(filter.condition(), instanceOf(expectedOperators[i]));
                BinaryComparison comparison;
                if (filter.condition() instanceof Not not) {
                    assertThat(not.children().get(0), instanceOf(Equals.class));
                    comparison = (BinaryComparison) (not.children().get(0));
                } else {