docs(readme): add example for service_account_impersonation_url in clientOptions (#1902)

* docs(readme): add service_account_impersonation_url example for AwsSupplier

The service_account_impersonation_url must be passed in the clientOptions
when using a custom AwsSupplier to avoid permissions errors. Updated the
README to include this information.

* docs(config): add service_account_impersonation_url example to .partials.yml

Updated the .partials.yml file to include the service_account_impersonation_url
configuration example, ensuring consistency with the README file.

Author: varun27896

Diff for: .readme-partials.yaml

@@ -434,6 +434,7 @@ body: |-
     audience: '//iam.googleapis.com/projects/$PROJECT_NUMBER/locations/global/workloadIdentityPools/$WORKLOAD_POOL_ID/providers/$PROVIDER_ID', // Set the GCP audience.
     subject_token_type: 'urn:ietf:params:aws:token-type:aws4_request', // Set the subject token type.
     aws_security_credentials_supplier: new AwsSupplier("AWS_REGION") // Set the custom supplier.
+    service_account_impersonation_url: 'https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/$EMAIL:generateAccessToken', // Set the service account impersonation url.
   }
 
   // Create a new Auth client and use it to create service client, i.e. storage.

Diff for: README.md

@@ -478,6 +478,7 @@ const clientOptions = {
   audience: '//iam.googleapis.com/projects/$PROJECT_NUMBER/locations/global/workloadIdentityPools/$WORKLOAD_POOL_ID/providers/$PROVIDER_ID', // Set the GCP audience.
   subject_token_type: 'urn:ietf:params:aws:token-type:aws4_request', // Set the subject token type.
   aws_security_credentials_supplier: new AwsSupplier("AWS_REGION") // Set the custom supplier.
+  service_account_impersonation_url: 'https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/$EMAIL:generateAccessToken', // Set the service account impersonation url.
 }
 
 // Create a new Auth client and use it to create service client, i.e. storage.