Add batch API support (Track v2)

[hownowstephen]

Summary

• Add Client#batch for POST /api/v2/batch (Track v2 API)
• Accepts an array of operations — identify, event, delete, etc. for people and objects
• Uses existing Track client auth (site_id + api_key)
• Pass-through design: no client-side validation of individual operations, API handles that

Closes #106. Requested by multiple users over 18+ months.

Test plan

• CI passes on Ruby 3.3, 3.4, 4.0, head
• Mixed operations sent correctly to /api/v2/batch
• Object operations work
• Empty/non-array input rejected
• Non-2xx responses raise InvalidResponse

---

Note

Low Risk
Adds a new API wrapper method and documentation/tests for posting batch operations; behavior is straightforward and limited to a new endpoint call with basic input validation.

Overview
Adds Client#batch to send Track v2 batch operations via POST /api/v2/batch, accepting an array of operations and rejecting empty/non-array inputs.

Updates the README with usage examples for batching identify/event/delete operations, and adds specs covering mixed person/object batches plus error handling for invalid inputs and non-2xx responses.

^{Reviewed by Cursor Bugbot for commit 3745567. Bugbot is set up for automated code reviews on this repo. Configure here.}