[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.

Start by locating the exported initializeApp and getApp entry points and their existing tests. The work is done when the new function returns an existing FirebaseApp or initializes one when none exists, with tests covering both cases.

Written by the indexing model from the issue text.