Add a stretched cluster conversion command.
Add a stretched cluster conversion command.

Add a new command `cluster.stretch` that wraps the API to convert
a vSAN cluster into a stretched vSAN cluster.

Co-authored-by: Jamie Monserrate <a72ab3fab7fa6bff65d3227933c4517ed682d196@vmware.com>

bruceadowns/govmomi,vmware/govmomi,vmware/govmomi,bruceadowns/govmomi,bruceadowns/govmomi,vmware/govmomi