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

[arayaryoma]

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: #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: arayaryoma@65be170

Describe alternatives you've considered
Not yet

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

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