Application Credentials are not fully parsed/validated during initializeApp

### [READ] Step 1: Are you in the right place?
Yes

### [REQUIRED] Step 2: Describe your environment

* Operating System version: Linux
* Firebase SDK version: 9.20
* Firebase Product: FCM
* Node.js version: v12.18.4
* NPM version: 6.14.6

### [REQUIRED] Step 3: Describe the problem

#### Steps to reproduce:

1. Copy JSON-encoded Google Application Credentials from a browser based tool like Bitwarden
2. Attempt to use the SDK with those credentials (valid JSON containing a poorly-encoded/formatted PEM)
3. Initialize the app without error
4. Send an FCM message, receive an error: ``` error: {
"library": "PEM routines",
"function": "get_name",
"reason": "no start line",
"code": "ERR_OSSL_PEM_NO_START_LINE"
}```

**What I expect to happen:**
I would like the error to be thrown when I `initializeApp` during the bootstrapping of our application so I can treat it as a configuration error, rather than an error we hit at runtime the first time that a message is sent.

#### Relevant Code:

```javascript
const FCM_TOKEN = JSON.parse(GOOGLE_APPLICATION_CREDENTIALS)

const serviceAccount = {
projectId: FCM_TOKEN.project_id,
clientEmail: FCM_TOKEN.client_email,
privateKey: FCM_TOKEN.private_key
}

// Shouldn't this to throw if the credentials cannot be parsed?
firebaseAdmin.initializeApp({
credential: firebaseAdmin.credential.cert(serviceAccount),
databaseURL: 'https://xxxx.firebaseio.com'
})
```