Add ability to configure custom HTTP headers at ChatClient.Builder

Currently, Spring AI only provides a straightforward way to set custom HTTP headers in certain provider-specific integrations (e.g., spring.ai.azure.openai.custom-headers for Azure OpenAI).
For other providers accessed via the ChatClient API — especially those using OpenAI-compatible endpoints like Google Gemini — there is no native mechanism to set provider-specific HTTP headers during client creation.

Expected Behavior
Enable adding custom HTTP headers directly when building a ChatClient.
For example:

ChatClient client = ChatClient.builder()
.baseUrl("https://generativelanguage.googleapis.com/v1beta/openai/")
.apiKey("...")
.customHeader("X-Server-Throughput-Key", "my-throughput-token")
.customHeader("Another-Header", "value")
.build();

Benefits:

Simplifies integrating with providers that require extra headers (e.g., Gemini throughput provisioning keys, experimental API )

Context

I work for enterprise system. We have reserved provisioning throughput on vertex ai Gemini. But can't use it natively with Spring AI

Start at ChatClient.Builder and trace how a built client creates HTTP requests for provider-compatible endpoints. Done means custom headers can be supplied during client creation and are included in requests, including multiple headers such as the Gemini throughput key.