[FR] Add a function that initializes an App or returns existing one

Is your feature request related to a problem? Please describe.
A function initializeApp throws an error if the app is already existed at the time. As current workaround, we need to handle thrown error.
(related: https://github.com/firebase/firebase-admin-node/issues/2111 )

Describe the solution you'd like
Add a function wrapping initializeApp and getApp, that returns existing FirebaseApp, or initialize a new FirebaseApp if the app doesn't exist.

my sample implementation: https://github.com/arayaryoma/firebase-admin-node/commit/65be1703a0c4f8ede7fd34c63d71e05b38f48efb

Describe alternatives you've considered
Not yet

Additional context
If this proposal is affirmed, I can open a PullRequest.

首先定位导出的 initializeApp 和 getApp 入口点及其现有测试。当新函数在已有 FirebaseApp 时返回它，或在不存在时初始化一个，并且测试覆盖这两种情况后，工作即完成。