Cannot call `getSignedUrl` when using authorized_user default credentials locally

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### [REQUIRED] Step 2: Describe your environment

* Operating System version: macOS 11.7.9
* Firebase SDK version: firebase-admin 11.11.0
* Firebase Product: Cloud Storage
* Node.js version: 18
* NPM version: 9.9.0

### [REQUIRED] Step 3: Describe the problem

#### Steps to reproduce:

What happened? How can we make the problem occur?
This could be a description, log/console output, etc.

I'm initializing like:

```
export const admin = firebaseAdmin.initializeApp({
credential: firebaseAdmin.credential.applicationDefault(),
...
```

but when I do that am unable to call `Bucket.file().getSignedUrl()` with the error:

```
Error: Cannot sign data without `client_email`
```

Looking into my local Firebase credentials, I notice there is no `client_email` there, but there is when using service accounts. I've found some issues around that seem to say this isn't possible with a personal service account ([example](https://github.com/googleapis/nodejs-storage/issues/360)).

If that's true, is it not possible at all to use my personal service account to work with files? Or is there a workaround that will enable this to work?

Start by tracing firebaseAdmin.credential.applicationDefault() and Bucket.file().getSignedUrl(), then compare the credential fields available for authorized_user credentials with the signing requirements. Done means establishing whether this credential type is supported, identifying a documented workaround, or defining the required SDK change.

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