fix: Made the apiKey parameter optional in the TavilyAnswerTool and TavilySearchResultsTool classes

[Nana-Kwame-bot]

• Description: This pull request introduces new documentation and updates for integrating Tavily tools, specifically the TavilyAnswerTool and TavilySearchResultsTool, into the project. It also includes some code refactoring to make the API key optional for these tools.

Documentation Updates:

• Added new documentation pages for TavilyAnswerTool and TavilySearchResultsTool in docs/modules/agents/tools and docs_v2/docs/05-integrations/tools directories. These pages provide detailed instructions on installation, configuration, usage, and error handling.
• Updated docs/_sidebar.md to include links to the new Tavily documentation pages.
• Updated docs_v2/sidebars.js to replace the old Tavily search page with the new Tavily answer and search results pages.
• Updated packages/langchain/README.md to include links to the new Tavily tools documentation.
• Updated packages/langchain_community/README.md to add DirectoryLoader and update the description for Tavily tools.

Code Refactoring:

• Made the apiKey parameter optional in the TavilyAnswerTool and TavilySearchResultsTool classes.
• Updated tests in packages/langchain_community/test/tools/tavily_test.dart and packages/tavily_dart/test/tavily_test.dart to reflect the optional apiKey parameter.
• Removed the api_key requirement from the tavily_openapi.yaml file.

• Issue: Tavily tools demand an api key although there's a baseUrl override. #644
• Dependencies: None
• Tag maintainer: @davidmigloz

[davidmigloz]

LGTM. Thanks for your work!