Support listing cluster credentials

[Westwooo]

Currently the shell can be used to create credentials on Capella clusters, we should also support listing creds through this endpoint: https://docs.couchbase.com/cloud/management-api-reference/index.html#tag/Database-Credentials/operation/listDatabaseCredentials.

Take a look at credentials create, we need to add a new command credentials that lists the database credentials that have been created on the active cluster.