Bed-4168: fix: Inaccurate asset group counts

[mistahj67]

Description

• Abstracted the asset group node fetch logic for both the /api/v2/asset-groups/{asset_group_id}/members endpoint as well as the RunAssetGroupIsolationCollections analysis step to ensure both counts will be in line post analysis.
• Added some missed licensing
• Updated go mod / sum

Motivation and Context

This PR addresses: BED-4168
It was determined that the numbers were still not in sync due to additional filtering steps included in the endpoint handler that were not in the analysis step.

How Has This Been Tested?

Tests + Locally hit via rest client against test data

Types of changes

• Chore (a change that does not modify the application functionality)
• Bug fix (non-breaking change which fixes an issue)

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