BED-4894: Posture API Spec

[computator]

Description

This adds documentation for the enterprise-only finding-trends and posture-history endpoints to the OpenAPI Spec.

Motivation and Context

This PR addresses: BED-4894

How Has This Been Tested?

Verified that the spec appears correctly in the VSCode "OpenAPI Preview" extension and that the JSON spec generation succeeds.

Types of changes

• Chore (a change that does not modify the application functionality)

Checklist:

• I have met the contributing prerequisites
  • Assigned myself to this PR
  • Added the appropriate labels
  • Associated an issue: Issues and Pull Requests README #672
  • Read the Contributing guide: https://github.com/SpecterOps/BloodHound/wiki/Contributing
• I have ensured that related documentation is up-to-date
  • Open API docs
  • Code comments (GoDocs / JSDocs)
• I have followed proper test practices
  • Added/updated tests to cover my changes
  • All new and existing tests passed

[sircodemane]

lgtm