Add a command for permission set group assignment and unassign...

[garychenming]

Is your feature request related to a problem? Please describe.
Add a command for permission set group assignment and unassign

Describe the solution you'd like

sfdx force:user:permset:assign -n permissionSetAPIName // assign a permission set
sfdx force:user:permset:assign -g permissionSetGroupAPIName // assign a permission set group

sfdx force:user:permset:unassign -n permissionSetAPIName // unassign a permission set
sfdx force:user:permset:unassign -g permissionSetGroupAPIName // unassign a permission set group