Add retry config option

[aminpaydar]

To add feature for configurable retry option.

#476

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[gemini-code-assist]

Code Review

This pull request updates copyright headers to 'Google LLC' across the SDK and bumps the version to 4.19.0. Significant additions include a new QueryUsers API for the Auth service (supporting both project and tenant levels), support for named Firestore databases via FirestoreWithDatabaseID, and a WithRetryConfig option for customizing HTTP client retries. A new AGENTS.md guide was also introduced. Feedback addressed minor typos in the documentation and recommended avoiding the mutation of input arguments within the QueryUsersRequest building logic to prevent unexpected side effects.

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AGENTS.md (24)

There is a typo here. Firbase should be Firebase.

-   **Error Handling:** Errors are handled using standard Go idioms. Firebase errors are defined in `internal/errors.go` however these errors can be further modified within each service. This modification is applied using that service's set `internal.HTTPClient.CreateErrFn` value.

AGENTS.md (72)

There is a typo here. barnch should be branch.

- When creating a new branch use the format `agentName-short-description`.

AGENTS.md (124)

It's a common convention to end files with a newline character. Please add one here.

- id: firebase-admin-go

auth/user_mgt.go (1122-1125)

This block modifies the original QueryUsersRequest passed by the caller by setting ReturnUserInfo if it's nil. This is a side effect that might be unexpected. It's generally better to avoid mutating input arguments. Consider creating a copy of the request struct q at the beginning of the build method and working with the copy instead.

For example:

func (q *QueryUsersRequest) build() interface{} {
    // ...
    req := *q // create a copy
    if req.ReturnUserInfo == nil {
        t := true
        req.ReturnUserInfo = &t
    }
    // ... then use req to build the final struct
}