Update: Added an optional endpoint to GCS filesystem schema

[NGizila]

This PR adds configurable custom GCS endpoint support. Users can override the default GCS API endpoint, mirroring the flexibility available in AWS and Azure, allowing the system to connect to S3NS (french provider "S3NS Trusted Cloud" compatible with GCS library) and other GCS-compatible storage backends.

The default is "storage.googleapis.com", but now user can overwrite it by specifying for example the endpoint "storage.s3nsapis.fr".

Please note, that currently in the code you will find a reference to forked sdk repository upgraded with "GCS custom endpoint".
I've created a corresponding Merge Request in sftpgo/sdk original project that add an endpoint line to GCS filesystem schema. If you approve SDK merge request I will be able to update it in current project.

Added changes are related to created issue by @sylguyot : Add endpoint parameter to GCS Storage backend

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[drakkan]

Hi, thanks for your contribution. Please sign our CLA so we can proceed with the review. That said, I'm a bit hesitant to review/merge this as I’m currently unable to test it myself