Generate signed URLs when using workload identity in Java

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*Is your feature request related to a problem? Please describe.*
[Storage] We are opening this Feature Request as we want to be able to easily generate signed URLs when using workload identity. We understand that this is a known missing feature and it is being worked on through other GitHub feature requests for .net [1] and ruby [2], but we want this functionality to also be available for java.

*Describe the solution you'd like*
We found a recent Google Cloud Collective response in a Stack Overflow issue [3] explaining the following: “External account credentials (Workload ID) are not supported as URL signers and you need to use the IAM service to sign the blob yourself. External account credentials are not currently supported for URL signing because it's not always possible to know client side which service account the credential maps back to, and that's a requirement (we would be calling the IAM service internally for this).”
We know this is already being worked on and there are some workarounds for .net [1] and ruby [2], therefore, we want to have a resolution/workaround on the Java side as well.

*Describe alternatives you've considered*
Tried using this example [4] by adapting it to Java, but started receiving the following exception:
```
Exception in thread "main" java.lang.IllegalStateException: Signing key was not provided and could not be derived
at com.google.common.base.Preconditions.checkState(Preconditions.java:512)
at com.google.cloud.storage.StorageImpl.signUrl(StorageImpl.java:709)
at com.ayla.cloud.gcs.controller.GCSController.getDownloadURL(GCSController.java:70)
at com.ayla.cloud.gcs.controller.GCSController.greeting(GCSController.java:42)
at com.ayla.cloud.gcs.controller.GCSController.main(GCSController.java:49)
```
This is the code being used:
```java
package com.ayla.cloud.gcs.controller;
import com.google.auth.oauth2.ComputeEngineCredentials;
import com.google.auth.oauth2.IdTokenCredentials;
import com.google.cloud.storage.BlobId;
import com.google.cloud.storage.BlobInfo;
import com.google.cloud.storage.Storage;
import com.google.cloud.storage.StorageOptions;
import java.io.IOException;
import java.net.URL;
import java.util.Arrays;
import java.util.concurrent.TimeUnit;

public class GCSController {
public String greeting() {
String bucketName = "";
String path = "example2/example-1001";
long ttl = 60;
String gcsSignatureVersion = "V4";
ComputeEngineCredentials caCreds = ComputeEngineCredentials.create();
String targetAudience = "https://example.com";
IdTokenCredentials tokenCredential = IdTokenCredentials.newBuilder() .setIdTokenProvider(caCreds)
.setTargetAudience(targetAudience)
.setOptions(Arrays.asList(ComputeEngineCredentials.Option.FORMAT_FULL)) .build();
URL url = getDownloadURL(bucketName, path, ttl, gcsSignatureVersion, tokenCredential);
return url.toString();
}

public static void main(String[] args) throws IOException {
System.out.println(new GCSController().greeting());
}

public URL getDownloadURL(String bucketName, String path, long ttl, final String gcsSignatureVersion, IdTokenCredentials tokenCredential) {
BlobInfo blobInfo = BlobInfo.newBuilder(BlobId.of(bucketName, path)).build();
Storage.SignUrlOption signUrlOption = null;

switch (gcsSignatureVersion) {
case "V2":
signUrlOption = Storage.SignUrlOption.withV2Signature();
break;
case "V4":
default:
signUrlOption = Storage.SignUrlOption.withV4Signature();
}
if (null == signUrlOption) {
return null;
}
Storage storage = StorageOptions.newBuilder().setCredentials(tokenCredential).build().getService();
return storage.signUrl(blobInfo, ttl, TimeUnit.MINUTES, signUrlOption);
}
}

```

*Additional context*
[1] https://github.com/googleapis/google-api-dotnet-client/issues/2410
[2] https://github.com/googleapis/google-cloud-ruby/issues/13307
[3] https://stackoverflow.com/a/76266912
[4] https://gist.github.com/jezhumble/91051485db4462add82045ef9ac2a0ec