Add Contacts API support

[teesofttech]

Summary

Implement the Termii Contacts API as its own SDK module/work item.

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

Endpoints

• Fetch contacts by phonebook ID: GET /api/phonebooks/{phonebook_id}/contacts
• Add single contact: POST /api/phonebooks/{phonebook_id}/contacts
• Upload multiple contacts via multipart CSV: POST /api/phonebooks/contacts/upload
• Delete contact: DELETE /api/phonebooks/{phonebook_id}/contacts

Acceptance criteria

• Add public contact client surface and request/response models.
• Wire the client into TermiiClient and dependency injection if applicable.
• Verify method, path, auth placement, query/body serialization, and response deserialization in unit tests.
• Cover multipart CSV upload without requiring live Termii credentials.
• Update README/docs examples for Contacts usage.

[teesofttech]

Implemented by #47.