Replace the Fetcher `dbname` parameter type to DatabaseName

We currently create a Fetcher using a string for the dbname parameter. This should be changed to use a DatabaseName instead.

Fetcher internally stores `dbname` as a string which is later used to create RemoteCommandRequest  which also takes a StringData as a parameter for dbName.

We can either complete this ticket and eventually convert the dbName.toString when passing it to the RemoteCommandRequest as part of this ticket.