fix(shell-api): Add support for running aggregate database with a single stage MONGOSH-1868

[gagik]

Running aggregate() on a database with a non-array pipeline will now run a single stage aggregation. This will make it consistent with db.{collection}aggregate behavior so

db.aggregate({$listLocalSessions: {}}

is same as

db.aggregate([{$listLocalSessions: {}}])

[gagik]

Now that I'm looking into documentation more, perhaps it'd make sense to completely sync up with .aggregate() with how db.coll.aggregate works so it's less confusing; i.e. you can also specify aggregation stages asaggregate({ stage1: ... }, {stage2: ...}) (in which case options will not be possible). It seems more reasonable to have a 1:1 mapping between these two aggregate functions' params.