Parse GOOGLE_APPLICATION_CREDENTIALS as JSON if it begins with '{'

**Is your feature request related to a problem? Please describe.**
I do not want to commit my credentials file to github, and when running my app inside another enviroment such as Heroku, i run into the trouble because even though i can specify ENV variables i must provide a file path for GOOGLE_APPLICATION_CREDENTIALS, which is inconvenient because im using git to deploy to heroku

**Describe the solution you'd like**
Parse GOOGLE_APPLICATION_CREDENTIALS as JSON if it begins with '{', otherwise assume is a file path.

**Describe alternatives you've considered**
Adding additional code that reads an env var which contains the credential json, this is inconvenient because i will need to copy and paste this code for all my projects that use firebase admin

```
fun createApp(): FirebaseApp {
val credentials = System.getenv("FIREBASE_CONFIG")
.byteInputStream()
.use { stream -> ServiceAccountCredentials.fromStream(stream) }
val config = FirebaseOptions.builder()
.setCredentials(credentials)
.build()
return FirebaseApp.initializeApp(config)
}
```

先找出 GOOGLE_APPLICATION_CREDENTIALS 的讀取位置，以及 Java SDK 中服務帳戶憑證的載入位置。追蹤現有的檔案路徑流程，然後確認以 '{' 開頭的值會被視為憑證 JSON，而其他值仍維持為檔案路徑；在不提交憑證檔案的情況下，兩種輸入形式都能正常運作即表示完成。

由索引模型根據 Issue 內容生成。