Storage SDK missing methods to reference multiple buckets

[samtstern]

[REQUIRED] Describe your environment

• Operating System version: macOS
• Browser version: N/A
• Firebase SDK version: exp
• Firebase Product: storage

[REQUIRED] Describe the problem

Originally reported here:
#4402

Steps to reproduce:

Relevant Code:

There seems to be no way to reproduce either of these snippets in the new modular SDK:

const storage = firebase.app().storage("gs://my-custom-bucket");

  const customApp = firebase.initializeApp({
    // ... custom stuff
  });

  // Get the default bucket from a custom firebase.app.App
  const storage = customApp.storage();

  // Get a non-default bucket from a custom firebase.app.App
  const storage = customApp.storage("gs://my-custom-bucket");

[hsubox76]

You can do:

const storage = getStorage(app, "gs://my-custom-bucket")

The second param is named url in the definition so I guess it should probably be named "bucket," and it doesn't have an accompanying jsdoc description so that should be added. Is that enough or am I missing anything else that would be needed?