Support Azure TokenCredential for authentication against Azure OpenAI

[asaikali]

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The current AzureOpenAI auto-configuration code assume that the users will provide a static api key if no key is set, then an exception is throw see https://github.com/spring-projects/spring-ai/blob/ca1949c8250187d94d735665b2044a844234aa51/spring-ai-spring-boot-autoconfigure/src/main/java/org/springframework/ai/autoconfigure/azure/openai/AzureOpenAiAutoConfiguration.java#L66

Azure supports a keyless authentication mode that uses Azure Active Directory and system identity. Some users of Azure will disable key based accessed for the Azure Open AI service and those users can't use spring-ai with azure open ai.

Todo:

• Enhance the the AzureOpenAiAutoConfiguration to support keyless authentication.
• Document how users can configure keyless authentication

Reference https://learn.microsoft.com/en-us/azure/developer/java/spring-framework/authentication

Resources:

• https://github.com/Azure-Samples/azure-openai-keyless-java