mongo: enable ShardedCluster + better code sharing for validation

[jgao54]

Change Streams API is documented to work out-of-the-box for sharded cluster. This PR updates the shared validation code to:

• check to make sure that the instance is either replica set or sharded cluster (previously, it ran replSetGetStatus to check this, we can instead use 'hello' command which is more general)
• add explicit validation for required roles (readAnyDatabase, clusterMonitor)
• better separation of concern by creating a commands.go file for all the db commands, while validation.go implements the validation logic -- will follow up with ClickPipes side as well.

Testing:

• tested replica set on Atlas
• test replica set on EC2
• test sharded cluster on Atlas
• test sharded clsuter on EC2