feat: add support for OAUTHBEARER token refresh callback

[kalavt]

This PR extends cppkafka::Configuration to support the sasl.oauthbearer.token.refresh.cb callback provided by librdkafka.

Motivation Mechanisms like AWS_MSK_IAM or generic OAUTHBEARER require the client to dynamically generate and refresh tokens when they expire. librdkafka handles this via a callback that requests a new token. Previously, cppkafka did not expose this callback, making it impossible to implement custom OAuth logic (like AWS SigV4 signing) purely in C++ without bypassing the wrapper.

Changes

• API: Added Configuration::set_oauthbearer_token_refresh_callback which accepts a std::function.
• Implementation: Implemented a proxy function oauthbearer_token_refresh_callback_proxy that correctly bridges the C-style librdkafka callback to the C++ function, properly casting the opaque handle.
• Documentation: Added docs/oauth_bearer_callback.md explaining how to use the new feature.
• Example: Added examples/oauth_example.cpp demonstrating a basic implementation.

Usage Example

config.set_oauthbearer_token_refresh_callback([](KafkaHandleBase& handle, const std::string& config) {
    // Generate token...
    rd_kafka_oauthbearer_set_token(handle.get_handle(), token_str.c_str(), expiry, ...);
});

This change is required for integrating AWS MSK IAM Authentication into ClickHouse.

[kalavt]

@antaljanosbenjamin for your review to address the comments
ClickHouse/ClickHouse#91118 (comment)

[antaljanosbenjamin]

Apart from this small change, the PR looks good. One question remains: do you mind also submitting the same PR to the upstream project or should I do that once this is merged?

The reason is we like to contribute back and it also help us keep our fork closer to the upstream one.

[antaljanosbenjamin]

Suggested change

	using OAuthBearerTokenRefreshCallback = std::function<void(KafkaHandleBase& handle,
	const std::string& oauthbearer_config)>;
	using OAuthBearerTokenRefreshCallback = std::function<void(KafkaHandleBase& handle,
	const std::string* oauthbearer_config)>;

Let's keep the semantic: if oauthbearer_config is not set, we should call the callback with nullptr.