Spring-ai Azure AI: Support OpenAI key and MS Entra auth

• Add azure-identity dependency
• Enable OpenAI API key and TokenCredential auth
• add the spring.ai.azure.openai.openai-api-key, non-azure property to allow connection to OpenAI service
• DefaultAzureCredential bean when the AZURE_CLIENT_ID, AZURE_TENANT_ID, AZURE_CLIENT_SECRET variables are present
• Update Azure Chat and Embedding docs
• Add integration tests for OpenAI connection

Resolves #260

Comment From: markpollack

removed use of DefaultAzureCredentialBuilder and associated dependencies. updated docs.

merged in 13a4e7651193163db996f5be54178ffecaf131af