### 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
assertEquals(
            new Enrich(
                EMPTY,
                PROCESSING_CMD_INPUT,
                new Literal(EMPTY, "countries", KEYWORD),
                new UnresolvedAttribute(EMPTY, "country_code"),
                null,
                List.of()
            ),
            processingCommand("enrich countries ON country_code")
        );
        expectError("from a | enrich countries on foo* ", "Using wildcards (*) in ENRICH WITH projections is not allowed [foo*]");
        expectError("from a | enrich countries on foo with bar*", "Using wildcards (*) in ENRICH WITH projections is not allowed [bar*]");
        expectError(
            "from a | enrich countries on foo with x = bar* ",
            "Using wildcards (*) in ENRICH WITH projections is not allowed [bar*]"
        );
        expectError(
            "from a | enrich countries on foo with x* = bar ",
            "Using wildcards (*) in ENRICH WITH projections is not allowed [x*]"
        );
    }
    public void testMvExpand() {
        LogicalPlan cmd = processingCommand("mv_expand a");
        assertEquals(MvExpand.class, cmd.getClass());
        MvExpand expand = (MvExpand) cmd;
        assertThat(expand.target(), equalTo(attribute("a")));
    }
    public void testUsageOfProject() {
        processingCommand("project a");