### 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/main/java/org/elasticsearch/xpack/esql/parser/LogicalPlanBuilder.java
import org.elasticsearch.xpack.ql.tree.Source;
import org.elasticsearch.xpack.ql.type.DataTypes;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Function;
import static org.elasticsearch.common.logging.HeaderWarning.addWarning;
import static org.elasticsearch.xpack.ql.parser.ParserUtils.source;
import static org.elasticsearch.xpack.ql.parser.ParserUtils.typedParsing;
import static org.elasticsearch.xpack.ql.parser.ParserUtils.visitList;
import static org.elasticsearch.xpack.ql.util.StringUtils.WILDCARD;
public class LogicalPlanBuilder extends ExpressionBuilder {
    public LogicalPlanBuilder(Map<Token, TypedParamValue> params) {
        super(params);
    }
    protected LogicalPlan plan(ParseTree ctx) {
        return ParserUtils.typedParsing(this, ctx, LogicalPlan.class);
    }
    protected List<LogicalPlan> plans(List<? extends ParserRuleContext> ctxs) {
        return ParserUtils.visitList(this, ctxs, LogicalPlan.class);
    }
    @Override
    public LogicalPlan visitSingleStatement(EsqlBaseParser.SingleStatementContext ctx) {
        return plan(ctx.query());
    }
    @Override