Support flushing of Capella buckets

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.

Start at the `buckets flush` command and inspect `cloud.rs`, then review the Capella V4 FlushBucket endpoint in the linked API documentation. Check how the cluster type is determined and identify where the Capella path should be selected. Done means Capella buckets can be flushed while existing local-bucket behavior remains intact.

Written by the indexing model from the issue text.