Parsing of SQL Functions body

It is my understanding that the current implementation of CREATE FUNCTION does not support the parsing of function blocks.

For instance, the following function body gets converted to a simple `DollarQuotedString`, as shown below. I need to be able to parse the function bodies to write any script that may be able to execute them, so I will be getting started doing that - is there any pull request that has already attempted this that I should be aware of?

```sql
CREATE FUNCTION emp_stamp() RETURNS trigger AS $emp_stamp$
BEGIN
-- Check that empname and salary are given
IF NEW.empname IS NULL THEN
RAISE EXCEPTION 'empname cannot be null';
END IF;
IF NEW.salary IS NULL THEN
RAISE EXCEPTION '% cannot have null salary', NEW.empname;
END IF;

-- Who works for us when they must pay for it?
IF NEW.salary < 0 THEN
RAISE EXCEPTION '% cannot have a negative salary', NEW.empname;
END IF;

-- Remember who changed the payroll when
NEW.last_date := current_timestamp;
NEW.last_user := current_user;
RETURN NEW;
END;
$emp_stamp$ LANGUAGE plpgsql;
```

```rust
Statement::CreateFunction {
or_replace: false,
temporary: false,
if_not_exists: false,
name: ObjectName(vec![Ident::new("emp_stamp")]),
args: None,
return_type: Some(DataType::Trigger),
function_body: Some(
CreateFunctionBody::AsBeforeOptions(
Expr::Value(
Value::DollarQuotedString(
DollarQuotedString {
value: "\n BEGIN\n -- Check that empname and salary are given\n IF NEW.empname IS NULL THEN\n RAISE EXCEPTION 'empname cannot be null';\n END IF;\n IF NEW.salary IS NULL THEN\n RAISE EXCEPTION '% cannot have null salary', NEW.empname;\n END IF;\n \n -- Who works for us when they must pay for it?\n IF NEW.salary < 0 THEN\n RAISE EXCEPTION '% cannot have a negative salary', NEW.empname;\n END IF;\n \n -- Remember who changed the payroll when\n NEW.last_date := current_timestamp;\n NEW.last_user := current_user;\n RETURN NEW;\n END;\n ".to_owned(),
tag: Some(
"emp_stamp".to_owned(),
),
},
),
),
),
),
behavior: None,
called_on_null: None,
parallel: None,
using: None,
language: Some(Ident::new("plpgsql")),
determinism_specifier: None,
options: None,
remote_connection: None
}
```

Beginne damit, den CREATE FUNCTION-Parser sowie die im Issue gezeigten Typen CreateFunctionBody, DollarQuotedString und Statement::CreateFunction zu lesen. Vergleiche die aktuelle opake Darstellung des Funktionskörpers mit dem angeforderten Parsing-Verhalten und identifiziere die Parser-Tests, die CREATE FUNCTION abdecken. Als abgeschlossen gilt die Aufgabe, wenn unterstützte Funktionsblöcke in einer für die nachgelagerte Skriptausführung geeigneten Darstellung geparst werden und Tests für das bereitgestellte PL/pgSQL-Beispiel vorhanden sind.

Vom Indexierungsmodell aus dem Issue-Text verfasst.