Add flexible API key management for OpenAI

Introduces a new API key interface that allows users to customize how
API keys are provided and managed in their Spring AI applications. This
change improves security and flexibility by:

- Adding core ApiKey interface and SimpleApiKey implementation
- Adding builder pattern for OpenAiApi creation
- Deprecating public constructors in favor of builder API (since 1.0.0.M6)
- Added docs

The new system enables users to implement their own key management
strategies while maintaining backward compatibility with property-based
configuration.

spring-ai-docs/src/main/antora/modules/ROOT/pages/api/chat/openai-chat.adoc

@@ -550,3 +550,46 @@ Follow the https://github.com/spring-projects/spring-ai/blob/main/models/spring-
 
 * The link:https://github.com/spring-projects/spring-ai/blob/main/models/spring-ai-openai/src/test/java/org/springframework/ai/openai/api/tool/OpenAiApiToolFunctionCallIT.java[OpenAiApiToolFunctionCallIT.java] tests show how to use the low-level API to call tool functions.
 Based on the link:https://platform.openai.com/docs/guides/function-calling/parallel-function-calling[OpenAI Function Calling] tutorial.
+
+== API Key Management
+
+Spring AI provides flexible API key management through the `ApiKey` interface and its implementations. The default implementation, `SimpleApiKey`, is suitable for most use cases, but you can also create custom implementations for more complex scenarios.
+
+=== Default Configuration
+
+By default, Spring Boot auto-configuration will create an API key bean using the `spring.ai.openai.api-key` property:
+
+[source,properties]
+----
+spring.ai.openai.api-key=your-api-key-here
+----
+
+=== Custom API Key Configuration
+
+You can create a custom instance of `OpenAiApi` with your own `ApiKey` implementation using the builder pattern:
+
+[source,java]
+----
+ApiKey customApiKey = new ApiKey() {
+    @Override
+    public String getValue() {
+        // Custom logic to retrieve API key
+        return "your-api-key-here";
+    }
+};
+
+OpenAiApi openAiApi = OpenAiApi.builder()
+    .apiKey(customApiKey)
+    .build();
+
+// Create a chat client with the custom OpenAiApi instance
+OpenAiChatClient chatClient = new OpenAiChatClient(openAiApi);
+
+----
+
+This is useful when you need to:
+
+* Retrieve the API key from a secure key store
+* Rotate API keys dynamically
+* Implement custom API key selection logic
+