Support new `/v1/convert/source/batch` endpoint

[edeandrea]

docling-serve v1.22.0 introduced a /v1/convert/source/batch endpoint.

https://developer.dcls.saas.ibm.com/api/convert-batch/ describes the endpoint. https://developer.dcls.saas.ibm.com/examples/batch/ describes how to use it.

Here is the full OpenAPI document - the api endpoint is in there:

api-1.yaml

[edeandrea]

This depends on #541