Namespace simplification pt3

[mlw]

The style guide prefers that a projects do not overly nest namespace names, and note that unless necessary, a single top-level namespace per project is the most effective strategy to limit exposure to collisions.

This is part 3 in a series of PRs to convert namespaces. This is being done piecemeal to keep PR sizes manageable. This PR converts the namespaces:

• santa::santad::event_providers::endpoint_security

Other namespaces will be converted in a future PR.

References:

• Namespaces style guide
• Namespace names style guide
• Tip of the Week #130: Namespace Naming
• Part 1: Adopt namespace naming guidelines - part 1 #1384
• Part 2: Namespace simplification pt2 #1385