Interactions API: Add `initiative` array to Interactions model

What changed

The Interactions API now accepts an optional initiative array on interaction payloads. This allows callers to associate an interaction with one or more campaigns (e.g. a texting or email campaign) at write time.

New field

"initiative": [
  {
    "type": "string",
    "id": "string"
  }
]

• type — the kind of initiative (e.g. "email", "sms")
• id — the external campaign identifier

The field is optional. Existing payloads without initiative are unaffected.

Example

{
  "contact_id": "abc123",
  "initiative": [
    {
      "type": "sms",
      "id": "campaign-456"
    }
  ]
}