Use positional name args instead of flags in `kn vsphere`

[embano1]

Is your feature request related to a problem? Please describe.
kn vsphere should follow similar CLIs, e.g. kubectl and kn and use positional args instead of flags (--name) for object names.

Describe the solution you'd like
kn vsphere source create mysource vs. kn vsphere source create --name mysource

Describe alternatives you've considered
keep as is

Additional context
Add any other context or screenshots about the feature request here.

[embano1]

Closing this since we don't have seen any user requests to change this. If it comes up, we can discuss the change.