[TypeScript] Initialize types too strict

[TheDuc]

Describe your environment

• Firebase SDK version: 8.3.0
• Firebase Product: firebase admin
• Node.js version: 10.16.0
• NPM version: 6.10.3

Describe the problem

When initialising firebase admin with standard downloaded service account json, typescript error incompatible types when "resolveJsonModule": true, in compiler

Relevant Code:

import * as admin from 'firebase-admin';

import * as serviceAccount from "path/to/serviceAccountKey.json";

admin.initializeApp({
  credential: admin.credential.cert(serviceAccount),     // <---- type error here
  databaseURL: "https://<DATABASE>.firebaseio.com"
});

tsconfig.json

"compilerOptions": {
    (...),
    "resolveJsonModule": true,
  },

[google-oss-bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.

[hiranya911]

Duplicate of #522. See the explanations and workarounds discussed there.