feat: add retriever and query engine implementations

[radu-mocanu]

Changes done:

• added context grounding retriever and query engine
• added context grounding docs
• better exception handling in runtime for workflow timeout exceptions and general python script issues
• added RAG sample + docs

To be merged after:
UiPath/uipath-python#410

Development Package

• Add this package as a dependency in your pyproject.toml:

[project]
dependencies = [
  # Exact version:
  "uipath-llamaindex==0.0.26.dev1000340105",

  # Any version from PR
  "uipath-llamaindex>=0.0.26.dev1000340000,<0.0.26.dev1000350000"
]

[[tool.uv.index]]
name = "testpypi"
url = "https://test.pypi.org/simple/"
publish-url = "https://test.pypi.org/legacy/"
explicit = true

[tool.uv.sources]
uipath-llamaindex = { index = "testpypi" }