Add support for firebase CI token

### The use case you're trying to solve

I want to deploy using firebase CI tokens rather than exporting a service account key for this task

### Change to the action that would solve that use case

Rather than [require a service account credential](https://github.com/FirebaseExtended/action-hosting-deploy/blob/0f248036885ae672f78587669fa2923f0ef6cac2/src/index.ts#L43) we could optionally have either a firebase ci token or a service account credential. Then when running firebase deploy, we could [append the token as necessary](https://firebase.google.com/docs/cli#:~:text=run%20all%20firebase%20commands%20with%20the%20--token%20token%20flag%20)

### Other considerations

Store the token as env variable.