Phase 2: Add new endpoint for returning Slack modal dropdown options

This is a non-user-facing piece of preliminary work for:

• Add assignees dropdown to incident slack modal
• Add labels dropdown to incident slack modal

As these dropdowns could many options, we'll want to load a subset of dropdown options based on user interaction.

Scope:

• token should be used to verify the source of the request, matching global_slack_handler.rb
• user/team info should be used to determine that the user has permissions to read the assignees
• Definition of action_id & block_id fields should be used to uniquely identify this dropdown menu (from any other potential dropdowns)
• Assignee list should update depending on project chosen
• Since the dropdown can only contain 100 items, typing should search through the available assignees/labels and prompt one that fits the search criteria