fix(shell-api): Align database and collection aggregate functions MONGOSH-1868

[gagik]

Looking into the documentation for both database and collection functions made me realize it'd be a lot clearer to align them completely. Instead of #2218 which would allow

db.aggregate({ $stage: 1 }, { options: true }) 
// => db.aggregate([{ $stage: 1 }], { options: true })

It'd be better to align with how db.{collection}.aggregate works i.e.

db.aggregate({ $stage: 1 }, { $stage2: true }) 
// => db.aggregate([{ $stage: 1 }, { $stage2: true })]);

This does mean if not using array, one cannot pass options but this is already the behavior of collection aggregate so it'd be best (and most intuitive) to keep it consistent.