Provide a way to update vSphere credentials on existing management clusters

Currently, if a vSphere password is changed after the management cluster is deployed, there is no easy way to update the secrets on the management cluster to update the password. This can cause issues when deploying new workload clusters, the expired creds are pulled from management cluster resulting in failures in cluster creation.

We should provide a way for users to update these credentials easily on management cluster either through a separate command or dynamically whenever a new workload cluster is deployed using the management cluster.

Start by tracing how vSphere credentials are stored on an existing management cluster and how those credentials are consumed during workload-cluster deployment. Compare the possible approaches described in the issue—a separate update command or dynamic refresh—and define the user-visible completion criteria, including successful deployment with changed credentials.

Written by the indexing model from the issue text.