Calls to shared.ParseTarget/ParseTargetWithDefault were swapped for
util.ParseTarget and util.ParseTargetWithDefaultOrganization, and code
now references target.Target instead of target.GroupName.

…how"

Add a VariableValue struct and two helper functions:
ToVariableValues converts a raw map[string]interface{} of variable
data into a slice of VariableValue, extracting value and secret flag.
FromVariableValues converts a slice of VariableValue back into the
map form expected by the API.

Add a new subcommand to manage variables within pipeline variable groups.
Introduces "azdo pipelines variable-groups variable list" to list variables
by variable group ID or name within a project (optionally prefixed by
organization).

…variable list'