Added support to TenableAD category APIs

[dheerajrp]

Description

Added Tenable.AD category APIs

Methods available:

• Retrieve all category instances - list
• Get category instance by id - details

Updated docs to support category API details.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Tests added to test all the category endpoints and their schema.
• Sphinx build for doc

Test Configuration:

• Python Version(s) Tested: 3.9
• Tenable.sc version (if necessary):

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules