Load .env file environment variables before instantiating openai clients

[EddieFahrenheit]

Description

This pull request allows for environment variables to be loaded from .env files before instantiating any of its OpenAI clients

Motivation and Context

Closes issue #431 Currently, the Camel repo doesn't search environment variables from .env files. This is important for extended implementations of Camel that depend on .env files to input their environment variables.

• [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)
• [x] 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)

Implemented Tasks

• [x] Install python-dotenv
• [x] Use load_dotenv() before every OpenAI client gets instantiated

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