Exit gracefully if no local content (rules, reference lists, data tables, or rule exclusions are found

Fixed bug: Convert the rule names to lowercase before checking for duplicate rule names. A DuplicateRuleNameError exception is raised if duplicate rule names are found.

Add commands to manage data tables in Google SecOps

Refactor rule manager as content manager and reorganize the various commands in preparation of adding support to manage data tables (and other content) via Google SecOps' API.

The original use case for this tool was to manage detection content (rules) via Google SecOps' API. The tool has since evolved to add support for managing other content such as reference lists and rule exclusions (and soon data tables).

This change does the following:

• Renames the rule manager tool to content manager
• Uses click to organize the CLIs commands, options, and arguments