Adds functionality to autocreate signed URLs for large file upload

[daschwanden]

This PR proposes an approach to autocreate signed URLs for large file upload with the Collect Large File flow.

• The approach taken is to autocreate the signed URLs in case the user (either via the UI or the API) creates a Collect Large File flow without submitting a signed URL.
• In case a Service Account (Server.signed_url_service_account_email) and a GCS Bucket (Server.signed_url_gcs_bucket_name) are configured and GRR is running on Google Kubernetes Engine (GKE) with Workload Identity Federation configured then the autocreation of the signed URL will succeed (see GRR documentation for more detail: https://grr-doc.readthedocs.io/en/latest/investigating-with-grr/collect-large-file.html).
• In case the above assumptions do not hold true then the Collect Large File flow will behave as it did before this functionality was introduced.
• Of course, the user is still free to create their own signed URL and run the Collect Large File flow as they did before this functionality was introduced.