Support GCS for storage

[NeckBeardPrince]

Describe the feature you'd like

Several years ago there was a PR to add support for GCS to use for storage. I'd like to request another feature review of adding GCS support.

Describe the benefits this would bring to existing BookStack users

Not every company, team, org uses AWS. Having more flexibility benefits everybody.

Can the goal of this request already be achieved via other means?

Yes, BookStack does support S3 compatible services like GCS. But it requires setting the GCS bucket to use a legacy access control called Fine-grained instead of the more secure Uniform. You also have to use Service account HMAC which uses Access Keys instead of either a normal GCP Service Account or, even better, Workload Identity.

Have you searched for an existing open/closed issue?

• I have searched for existing issues and none cover my fundamental request

How long have you been using BookStack?

Under 3 months

Additional context

#5262 and #763 would also be great so that buckets don't need to be public.