Validate the OpenAI API key format (#1635)

* Validate the OpenAI API key format

Increase the amount of internal validation for OpenAI API keys. The intent is
to shorten the debugging loop in case of typos. The changes do *not* add
validation for Azure OpenAI API keys.

* Add the validation in `__init__` of `OpenAIClient`.

* Introduce the `MOCK_OPEN_AI_API_KEY` constant for testing.

*  Add unit test coverage for the `is_valid_api_key` function.

* Validate the OpenAI API key format

Increase the amount of internal validation for OpenAI API keys. The intent is
to shorten the debugging loop in case of typos. The changes do *not* add
validation for Azure OpenAI API keys.

* Add the validation in `__init__` of `OpenAIClient`.

* Introduce the `MOCK_OPEN_AI_API_KEY` constant for testing.

*Add unit test coverage for the `is_valid_api_key` function.

* Log a warning when register a default client fails.

* Validate the OpenAI API key format

Increase the amount of internal validation for OpenAI API keys. The intent is
to shorten the debugging loop in case of typos. The changes do *not* add
validation for Azure OpenAI API keys.

* Add the validation in `__init__` of `OpenAIClient`. We'll log a
  warning when the OpenAI API key isn't valid.

* Introduce the `MOCK_OPEN_AI_API_KEY` constant for testing.

* Add unit test coverage for the `is_valid_api_key` function.

* Check for OpenAI base_url before API key validation

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Co-authored-by: Chi Wang <wang.chi@microsoft.com>