feat: Refactor OpenAIFunction to FunctionTool

[tom-doerr]

Description

I renamed the OpenAIFunction class to FunctionTool and created a new OpenAIFunction class that throws a deprecation warning when initialized.

Motivation and Context

closes #858

• [x] I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds core functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (update in the documentation)
• [ ] Example (update in the folder of example)

Checklist

Go over all the following points, and put an x in all the boxes that apply. If you are unsure about any of these, don't hesitate to ask. We are here to help!

• [x] I have read the CONTRIBUTION guide. (required)
• [x] My change requires a change to the documentation.
• [x] I have updated the tests accordingly. (required for a bug fix or a new feature)
• [x] I have updated the documentation accordingly.