feat: Add unified batch processing API

- [x] I have looked for existing issues (including closed) about this

## Feature Request
e.g. https://platform.claude.com/docs/en/build-with-claude/batch-processing
https://ai.google.dev/gemini-api/docs/batch-api
https://platform.openai.com/docs/guides/batch

### Motivation
To send asynchronous groups of requests with 50% lower costs, a separate pool of significantly higher rate limits, and a clear 24-hour turnaround time. The service is ideal for processing jobs that don't require immediate responses.

### Proposal
Not as of now. I saw there is already JSONL decoder here so might be a starting point https://github.com/0xPlaygrounds/rig/blob/a9be75857e8b90cb24cefa0f509a5af48e90da97/rig/rig-core/src/providers/anthropic/decoders/jsonl.rs#L1

### Alternatives
No

The issue mentions a JSONL decoder in rig/rig-core/src/providers/anthropic/decoders/jsonl.rs as a starting point. Research existing batch processing APIs from Claude, Gemini, and OpenAI for design patterns. The work involves designing a new asynchronous API for batch requests, likely touching core provider interfaces and rate limiting. 'Done' means a fully specified proposal and likely a prototype implementation.

Scritto dal modello di indicizzazione a partire dal testo della issue.