feat(campaigns): start_date for threat-actor-generated campaigns

Backend:
- campaign_service.generate_campaign_from_threat_actor: accept optional
  start_date kwarg and set it on the Campaign model
- campaigns router: new GenerateFromActorPayload schema, /from-threat-actor
  endpoint now accepts optional body with start_date

Frontend:
- generateCampaignFromThreatActor API: accept optional options param
- Generate Campaign modal: date picker + warning message, same UX as the
  manual create form

frontend/src/api/campaigns.ts

@@ -155,8 +155,14 @@ export async function getCampaignProgress(campaignId: string): Promise<CampaignP
 }
 
 /** Generate a campaign from a threat actor. */
-export async function generateCampaignFromThreatActor(actorId: string): Promise<Campaign> {
-  const { data } = await client.post<Campaign>(`/campaigns/from-threat-actor/${actorId}`);
+export async function generateCampaignFromThreatActor(
+  actorId: string,
+  options?: { start_date?: string },
+): Promise<Campaign> {
+  const { data } = await client.post<Campaign>(
+    `/campaigns/from-threat-actor/${actorId}`,
+    options ?? {},
+  );
   return data;
 }