Add tab completer for Resource Group

[maddieclayton]

Description

Add a resource group argument completer for appropriate cmdlets once work on #3808 is complete.

• Create tab completer for Resource Group by obtaining all resource groups for the current subscription
  • Ensure default resource group is first in list
  • Limit to 50 results
• Add tab completer to select cmdlets with ResourceGroup parameter (TBD)
• Add tests to verify all possible scenarios (no resource groups, too many resource groups, default set and not set)