Add end-to-end tests for completion and stand-alone rag pipelines

[dagardner-nv]

Description

• Add end-to-end tests for completion and stand-alone rag pipelines
• Optionally support openai as a backend for the rag pipeline
• Add --embedding_size to stand-alone rag pipeline, removing previously hard-coded value
• Fix evaluation of stop_after argument in RSS source stage.
• Relocate some fixtures from tests/llm/services/conftest.py to tests/llm/conftest.py
• Currently checks are failing for examples/llm/rag/persistant_pipeline.py, due to SplitStage not being updated.

By Submitting this PR I confirm:

• I am familiar with the Contributing Guidelines.
• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.