### 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
"row x = is_null(f)",
            "line 1:10: is_null function is not supported anymore, please use 'is null'/'is not null' predicates instead"
        );
    }
    public void testMetadataFieldOnOtherSources() {
        expectError(
            "row a = 1 [metadata _index]",
            "1:11: mismatched input '[' expecting {<EOF>, PIPE, 'and', COMMA, 'or', '+', '-', '*', '/', '%'}"
        );
        expectError("show functions [metadata _index]", "line 1:16: mismatched input '[' expecting {<EOF>, PIPE}");
        expectError(
            "explain [from foo] [metadata _index]",
            "line 1:20: mismatched input '[' expecting {PIPE, COMMA, OPENING_BRACKET, ']'}"
        );
    }
    public void testMetadataFieldMultipleDeclarations() {
        expectError("from test [metadata _index, _version, _index]", "1:40: metadata field [_index] already declared [@1:21]");
    }
    public void testMetadataFieldUnsupportedPrimitiveType() {
        expectError("from test [metadata _tier]", "line 1:22: unsupported metadata field [_tier]");
    }
    public void testMetadataFieldUnsupportedCustomType() {
        expectError("from test [metadata _feature]", "line 1:22: unsupported metadata field [_feature]");
    }
    public void testMetadataFieldNotFoundNonExistent() {
        expectError("from test [metadata _doesnot_compute]", "line 1:22: unsupported metadata field [_doesnot_compute]");
    }
    public void testMetadataFieldNotFoundNormalField() {
        expectError("from test [metadata emp_no]", "line 1:22: unsupported metadata field [emp_no]");
    }