feat(fcm): add retry config client option support

[aminpaydar]

Allow callers to pass retry policy through option.ClientOption so Messaging can honor custom retry behavior from NewApp options. When no retry option is supplied the default retry config is still applied.

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[gemini-code-assist]

Code Review

This pull request introduces the ability to configure HTTP retry behavior for Firebase service clients by adding a public WithRetryConfig option and refactoring the internal HTTP client creation logic. The messaging service has been updated to utilize these configurations. Feedback was provided regarding the cloneHTTPClient function, which performs a shallow copy that may be fragile if default options are added to the internal client in the future.