OpenAI API key format validation

[marklysze]

Why are these changes needed?

OpenAI's API key has changed in format recently and the validation function in OpenAI doesn't accurately validate the format.

OpenAI has not published the format of the key, so information gathered so far is that it can start with:
sk-
sk-proj-
sk-None-
sk-svcaat-

This is followed by segments of alphanumeric characters (including underscores) separated by dashes.

As the exact format is not defined, the regular expression in the validation function will be expanded to support keys:

• starting with sk-
• followed by at least 48 alphanumeric characters (including underscores and dashes)

The tests have been updated.

Related issue number

Closes #26

Checks

• I've included any doc changes needed for https://autogen-ai.github.io/autogen/. See https://autogen-ai.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[0xdre]

LGTM

[BlocUnited]

I tested this PR within my use case, where I run Autogen using uvicorn. The API key validation warning issue was resolved, and everything works as expected now. Thanks for the fix!