Rename resume_state to pipeline_snapshot in breakpoints

Once the Agent breakpoints [PR](https://github.com/deepset-ai/haystack-experimental/pull/328) is merged, we should rename the resume_state parameters and the broader pipeline_state object to pipeline_snapshot.
This renaming should be completed before integrating the breakpoints feature into Haystack.

Reference: https://github.com/deepset-ai/haystack-experimental/pull/317#discussion_r2142844865