feat: Add support for CloudBilling resource

[shravankshenoy]

Description

Issue #247 . Adds support for cloudbilling resource

Changes Made

• Created new client factory in client package : cloud_billing_client.py
• Created new crawler factory in the crawler package : cloud_billing_account_crawler.py
• In the scanner.py file, included the crawler in the crawl loop and updated the crawl_client_map.
• Modified example.config
• Added unit tests in test_unit.py
• Created bootstrap shell script to enable cloudbilling API (and code to link billing account to current project if required)
• Added sample output for testing

Note

Unlike most other resources, a single project can only have 1 billing account associated with it. Programatically, this would mean an if condition is used instead of a while loop in the crawler as only one resource is returned for a given project.