Add "Update Asset Group Selectors" endpoint

[maffkipp]

Description

• Adds the POST /v2/asset-groups/{asset_group_id}/selectors endpoint into BHCE
• Fixes a bug that caused this endpoint to always return empty lists of selectors
• Updates the documentation to reflect the correct request/response types for this endpoint

Motivation and Context

We are building a new asset group management page, which will need the ability to add and remove asset group selectors.

How Has This Been Tested?

• This handler has existing tests that were ported over along with it

Screenshots (if appropriate):

Types of changes

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

Checklist:

• Documentation updates are needed, and have been made accordingly.
• I have added and/or updated tests to cover my changes.
• All new and existing tests passed.
• My changes include a database migration.

[superlinkx]

Looks good!