Phase 3: Add labels dropdown to Slack incident creation modal

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

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

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

Scope:

• Add multi_external_select type dropdown for labels to slack modal json

Supplemental details:

• docs on external selects