Support `--only` flag in `firebase init` for consistency with `firebase deploy`

### Description

When running:

```sh
firebase init --only hosting
```

I get the following error:

```
error: unknown option '--only'
```

This was unexpected, since many Firebase developers (myself included) are used to running:

```sh
firebase deploy --only
```

It would be great if `firebase init` also supported the `--only` flag to align with this usage pattern. Even if it's just syntactic sugar for `firebase init hosting`, it would make the CLI feel more consistent and intuitive.

### Steps to Reproduce

1. Run `firebase init --only hosting`
2. Observe the error

### Expected Behavior

`firebase init --only hosting` should behave the same as `firebase init hosting`.

### Actual Behavior

The CLI throws an error saying `--only` is an unknown option.

### Additional Context

This might help reduce friction for developers transitioning between `deploy` and `init` commands.

I initially posted this on socials, where team member @mbleigh asked me to file a feature request: https://x.com/mbleigh/status/1915147194069557418