Phase 3: Add assignees dropdown to Slack incident creation modal

After "Add /gitlab incident declare slack slash command" is completed, we want to add a dropdown for incident assignees.

Here's how that would look in the incident creation modal:

Blocked by: "Add endpoint to return dropdown menu options"

Scope:

• Add external_select type dropdown for assignees to slack modal json

Supplemental details:

• docs on external selects