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
}
```

Commencez par lire le parseur de CREATE FUNCTION ainsi que les types CreateFunctionBody, DollarQuotedString et Statement::CreateFunction présentés dans l’issue. Comparez la représentation opaque actuelle du corps de fonction avec le comportement d’analyse demandé et identifiez les tests du parseur couvrant CREATE FUNCTION. Le travail est considéré comme terminé lorsque les blocs de fonction pris en charge sont analysés dans une représentation adaptée à l’exécution ultérieure de scripts, avec des tests pour l’exemple PL/pgSQL fourni.

Rédigé par le modèle d'indexation à partir du texte de l'issue.