Currently, we have the following commands/structs:

• create: command invoked to create a collection
• ShardCollection: struct used for the shardCollection command (client->router)
• CreateCollectionRequest: struct used for the _shardsvrCreateCollection command (router->shard)

All of the above commands/structs contain some duplicated fields (with the same meaning and name) and some other fields (not duplicated).

Purpose of this ticket is to:

• Unify the structs used for shardCollection and _shardsvrCreateCollection (if possible, they seem to have the exact same fields)
• Extract a struct containing all fields of the create command
• Chain the new struct to the shardCollection commands (removing duplicates)

This is a step towards having a unique code path for creating/sharding a collection.