feat: In breakpoints, add a callable func to `save_state` for passing `pipeline_snapshot` externally

[Amnah199]

In the early design of breakpoints, we had discussions about adding callback_fun in save_state while breaking the pipeline. This would provide the users a hook to inject custom behavior—for instance, to propagate the pipeline_snapshot externally via a RESTful API or HTTP request.

Related discussion: https://github.com/deepset-ai/haystack-experimental/pull/317#discussion_r2142574323