Add PR help message functionality and update dependencies

- Implement PRHelpMessage class to provide AI-powered assistance for pull requests.
- Add methods for similarity search using local, S3, and Pinecone databases.
- Update `requirements.txt` to include new dependencies for langchain and chromadb.
- Modify `configuration.toml` to include `force_local_db` setting for PR help.
- Update `aiohttp` and `openai` package versions.

pr_agent/settings/configuration.toml

@@ -183,6 +183,7 @@ enable_help_text=true
 final_update_message = false
 
 [pr_help] # /help #
+force_local_db=false
 
 [pr_config] # /config #