Complete Campaign API support

[teesofttech]

Summary

Complete the Termii Campaign API as its own SDK module/work item.

Docs: https://developers.termii.com/campaign
Parent split from: #42

Endpoints

• Send campaign: POST /api/sms/campaigns/send
• Fetch campaigns: GET /api/sms/campaigns
• Fetch campaign history: GET /api/sms/campaigns/{campaign_id}
• Retry failed campaign: PATCH /api/sms/campaigns/{campaign_id}

Acceptance criteria

• Extend the campaign client beyond phonebook CRUD with campaign send/list/history/retry operations.
• Add request/response models for campaign creation, list records, history details, and retry responses.
• Verify method, path, auth placement, query/body serialization, and response deserialization in unit tests.
• Keep existing phonebook behavior and tests intact.
• Update README/docs examples for Campaign usage.

[teesofttech]

Implemented by #48.