Support Azure TokenCredential for authentication against Azure OpenAI

[asaikali]

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

spring-ai/spring-ai-spring-boot-autoconfigure/src/main/java/org/springframework/ai/autoconfigure/azure/openai/AzureOpenAiAutoConfiguration.java

Line 66 in ca1949c

public OpenAIClientBuilder openAIClientBuilder(AzureOpenAiConnectionProperties connectionProperties) {

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

[KKulma]

I second this—at the very least, please provide documentation on how to update relevant scripts to enable authentication via Service Principal. We're wrestling with it at the moment, and autoconfiguration is stubbornly hard to override!