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
}
```

Comienza leyendo el parser de CREATE FUNCTION y los tipos CreateFunctionBody, DollarQuotedString y Statement::CreateFunction mostrados en el issue. Compara la representación opaca actual del cuerpo de la función con el comportamiento de parsing solicitado e identifica las pruebas del parser que cubren CREATE FUNCTION. Se considera terminado cuando los bloques de función compatibles se analizan en una representación adecuada para la ejecución posterior de scripts, con pruebas para el ejemplo de PL/pgSQL proporcionado.

Escrito por el modelo de indexación a partir del texto del issue.