Support flushing of Capella buckets

[Westwooo]

The buckets flush command currently only works against locally running Couchbase buckets. We need to add support for flushing data from Capella buckets.

To do this check if the cluster the command is being run against is Capella (if cluster.clsuter_type() == Provisioned) and if so then we need to use the Capella V4 flush endpoint: https://docs.couchbase.com/cloud/management-api-reference/index.html#tag/Buckets-Scopes-and-Collections/operation/FlushBucket to flush the bucket.

This should be done by adding a flush_bucket function to cloud.rs and call when appropriate.