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)
}
```

Comece localizando onde GOOGLE_APPLICATION_CREDENTIALS é lido e onde as credenciais da conta de serviço são carregadas no Java SDK. Rastreie o fluxo existente de caminhos de arquivo e, em seguida, verifique se os valores que começam com '{' são tratados como JSON de credenciais, enquanto os outros valores continuam sendo caminhos de arquivo; está concluído quando ambas as formas de entrada funcionarem sem fazer commit de um arquivo de credenciais.

Escrita pelo modelo de indexação a partir do texto da issue.