Add appdist group and tester list commands

[duncan-google]

Description

Adds the following CLI commands for App Distribution:

• firebase appdistribution:testers:list
• firebase appdistribution:group:list

Scenarios Tested

• List all testers
• List all testers in a group
• List all groups

Sample Commands

List all AppDist testers:

firebase appdistribution:testers:list --project my-project

List all AppDist testers in the alpha-team group:

firebase appdistribution:testers:list alpha-team --project my-project

List all AppDist groups:

firebase appdistribution:group:list --project my-project 

List all AppDist groups, providing JSON as output:

firebase appdistribution:group:list --project my-project --json