feature suggestion: expose fetchone() and fetchall() methods on the Engine and SAConnection

Similarly to `engine.scalar()` and `connection.scalar()`, it would be convenient if there were `fetchone()` and `fetchall()` methods as well, as shortcuts for:

```
result = await connection.execute(...)
data = await result.fetchall()
```

I know that sqlalchemy's engine and connection don't have those methods, but since aiopgsa is async, simply chaining the method calls is significantly messier and less clear:

```
data = await (await connection.execute(...)).fetchall()
```

Happy to open a PR if this is something that would get merged.