events: Add custom code for event ingestion

[Yopi]

• Adds syntactic sugar to ingest costs and llm metadata
• Adds an easy way to ingest events and child-events (as spans)

[Yopi]

  // Raw API - existing
  await polar.events.ingest({ events: [{ name: "evt", customerId: "c_1", metadata: { "_cost": { amount: 100, currency: "usd" } } }] });

  // Sugar - enriches cost/llm into metadata automatically
  await polar.events.sendEvents([
    { name: "evt", externalCustomerId: "user_123", cost: { amount: 100, currency: "usd" }, llm: { ... } },
  ]);

  // Span - creates parent, returns client that auto-applies parentId + customer
  const span = await polar.events.withSpan({ name: "request", externalId: "req_1", externalCustomerId: "user_123" });
  await span.sendEvents([
    { name: "child.step", cost: { amount: 50, currency: "usd" } },
  ]);