[firebase_storage] Check if a file exists without throwing exceptions

## What feature would you like to see?

Currently there is no function or method to cleanly check if a file exits in Firebase Storage.

This feature request is derived from Flutterfire [issue 11620](https://github.com/firebase/flutterfire/issues/11620).

There are many ways that have been proposed to check if a file exists, which all consist on try-catch. This however results in the SDK throwing exception logs into the console.

## How would you use it?

Have an exists async function in the Reference object which will return true or false depending on whether the file exists in Firebase Storage.