Support a remote OpenAI-compatible API backend for chart extraction (ChartExtractionApiOptions)

### Requested feature
Allow Docling's chart extraction enrichment (`do_chart_extraction`) to run against a remote OpenAI-compatible Chat Completions endpoint (e.g. vLLM, Ollama, TGI, LM Studio) — the same way picture description already supports remote inference via `PictureDescriptionApiOptions` + `enable_remote_services=True`.

Concretely: an API-backed variant of `ChartExtractionModelOptions` (e.g. `ChartExtractionApiOptions`) exposing url, headers, params, prompt, timeout, and concurrency, so the chart→ CSV/code/summary step can be offloaded to a self-hosted VLM behind an HTTP API instead of loading ibm-granite/granite-vision-*-chart2csv locally through transformers.

Locate ChartExtractionModelOptions and compare its chart-extraction entry point with PictureDescriptionApiOptions and enable_remote_services=True. Implement the requested ChartExtractionApiOptions fields—url, headers, params, prompt, timeout, and concurrency—and verify do_chart_extraction can use a remote OpenAI-compatible endpoint without loading the local chart model.

Written by the indexing model from the issue text.