Phase 2: Add `status` field to Slack incident creation modal

After "Add /gitlab incident declare slack slash command" is completed, we want to add a dropdown for incident status. The incident should be initialized with the provided status (should not trigger an update event for webhooks). Here's how that dropdown would look in the incident creation modal:

Note: This should be behind the :incident_creation_slash_command feature flag.

Scope:

• Add dropdown to slack modal json
• Parse status value from modal submission
• Include status in issue creation

Supplemental details:

• Both app/services/issues/create_service.rb & app/services/incident_management/issuable_escalation_statuses/create_service.rb will need to accept attributes for IssuableEscalationStatus records