Add better support for using OpenAI subscriptions

[hootanht]

Fixes #2

Add support for configuring OpenAI subscriptions directly.

• README.md:
  • Add instructions for configuring OpenAI subscriptions.
  • Update example code to include OpenAI configuration.
• src/skUnit/Asserts/SemanticAssert.cs:
  • Add a new constructor to accept OpenAI API key directly.
• src/skUnit/Asserts/SemanticKernelAssert_Initialize.cs:
  • Add a new constructor to accept OpenAI API key directly.
• src/skUnit.Tests/SemanticAssertTests/SemanticAssertTests.cs:
  • Add tests for the new OpenAI constructor.
  • Update existing tests to include OpenAI configuration.

[mehrandvd]

@hootanht wow... thank you for contributing :)

[mehrandvd]

The latest changes already add support for OpenAI subscription and support for IChatClient. Now you can pass an IChatClient which can be instantiated using any provider.

So, I'm closing this PR.