Add GetQuotaFailure to the public API

[justinattw]

NOTE: This PR is based on #737
For now, I've opened a PR in justinattw#1 that reviews the change into the dependent PR.

Changes
Based on (and requires first merging of) #737
Closes #736

https://firebase.google.com/docs/reference/fcm/rest/v1/ErrorCode specifies that google.rpc.QuotaFailure is returned with FCM.

We expose this via the new public API GetQuotaFailure(err *errorutils.FirebaseError) *QuotaFailure method.

I also refactored the messaging.go error-related code bits into messaging_errors.go.

[gemini-code-assist]

Summary of Changes

Hello @justinattw, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the error handling capabilities of the Firebase Admin Go SDK, particularly for Firebase Cloud Messaging. It introduces a new public API to programmatically access detailed quota failure information, which is crucial for applications dealing with rate limits. Concurrently, it undertakes a substantial refactoring effort by moving common error types and utilities to a dedicated errorutils package, ensuring a more standardized and accessible error reporting mechanism throughout the SDK.

Highlights

• Public API for Quota Failures: Introduced a new public API function, GetQuotaFailure(err *errorutils.FirebaseError) *QuotaFailure, in the messaging package. This function allows developers to extract detailed quota violation information from Firebase Cloud Messaging (FCM) errors, providing insights into which specific quotas were exceeded.
• Error Handling Refactoring: The core FirebaseError type and ErrorCode constants have been moved from the internal package to a new, publicly accessible errorutils package. This change promotes consistency and reusability of error types across the SDK.
• Messaging Error Code Consolidation: FCM-specific error handling logic and related helper functions (e.g., IsInternal, IsQuotaExceeded) have been extracted from messaging.go into a new file, messaging_errors.go, improving modularity and maintainability.
• FirebaseError Field Renaming: The String field within the FirebaseError struct has been consistently renamed to Message across various service implementations (Auth, DB, IID, Remote Config) to better reflect its purpose.

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[gemini-code-assist]

Code Review

This pull request introduces a significant and beneficial refactoring of the error handling mechanism by exposing error-related types and functions in a new public errorutils package. This makes the error handling more transparent and idiomatic for users of the SDK. The core change of replacing the internal String field with a public Message field in FirebaseError is consistently applied across the codebase.

Additionally, this PR adds a new public API, GetQuotaFailure, to the messaging package, which allows developers to inspect detailed quota failure information from FCM. The related error handling logic has been cleanly refactored into a new messaging_errors.go file.

The changes are well-implemented and include comprehensive tests. I have a couple of suggestions to further improve the ergonomics and maintainability of the new additions.