[Sample] Add a sample which converts a langchain app to flex flow with custom connection

[D-W-]

Description

Please add an informative description that covers that changes made by the pull request and link all relevant issues.

This pull request introduces a new feature to the langchain-eval example in the flex-flows directory. The new feature includes a workflow for evaluating language chains, as well as changes to the README, data, and requirements files. The most significant changes are:

1. A new GitHub workflow file .github/workflows/samples_flexflows_langchaineval_langchaineval.yml was added. This workflow is scheduled to run daily and on pull requests that modify specific paths. It sets up a Python 3.9 environment, installs requirements, and runs a notebook test, among other steps.

2. The examples/README.md file was updated to include a link to the langchain-eval.ipynb notebook and its corresponding workflow badge.

3. Two new data entries were added to the examples/flex-flows/langchain-eval/data.jsonl file.

4. The examples/flex-flows/langchain-eval/flow.flex.yaml file was added, which contains the schema and entry point for the new feature.

5. A new Python file examples/flex-flows/langchain-eval/flow.py was added. This file contains the LangChainEvaluator class, which is used to evaluate language chains.

6. The examples/flex-flows/langchain-eval/langchain-eval.ipynb notebook was added. This notebook provides a tutorial on how to use the new feature.

7. The examples/flex-flows/langchain-eval/requirements.txt file was updated to include the promptflow and langchain packages, as well as the python-dotenv package.

All Promptflow Contribution checklist:

• [ ] The pull request does not introduce [breaking changes].
• [ ] CHANGELOG is updated for new features, bug fixes or other significant changes.
• [ ] I have read the contribution guidelines.
• [ ] Create an issue and link to the pull request to get dedicated review from promptflow team. Learn more: suggested workflow.

General Guidelines and Best Practices

• [ ] Title of the pull request is clear and informative.
• [ ] There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• [ ] Pull request includes test coverage for the included changes.