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

[brianmaissy]

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.

[brianmaissy]

@Pliner what do you think about this one?