feat: add support for custom storage JSON endpoint

[Tchebychev]

Problem

We are using GitLab with S3NS (Thales' trusted cloud, which offers a Google Cloud Storage-compatible API).
GitLab uses the fog-google gem to interact with Google Cloud Storage buckets.

However, we faced connection issues because fog-google currently hardcodes the Google Cloud Storage API endpoint (https://www.googleapis.com/storage/) and the public bucket URL (https://storage.googleapis.com/).
It was not possible to redirect these calls to the S3NS endpoint via configuration, making it impossible to use fog-google (and thus GitLab) with S3NS or other GCS-compatible sovereign cloud providers.

Solution

This PR introduces support for configuring a custom API endpoint and respecting the host configuration for public URLs.

It adds a new configuration option :google_json_root_url to override the default API root URL.
It also updates Real and Mock classes, as well as Directory and File models, to generate URLs dynamically based on the configured :host, :scheme, and :port instead of using the hardcoded Google domains.

Changes

• lib/fog/google/storage/storage_json.rb: Added :google_json_root_url to recognized options.
• lib/fog/google/storage/storage_json/real.rb: Updated initialization to use google_json_root_url if provided, and added a bucket_base_url helper method.
• lib/fog/google/storage/storage_json/mock.rb: Implemented bucket_base_url for consistency in tests.
• lib/fog/google/storage/storage_json/models/directory.rb & file.rb: Updated public_url methods to use bucket_base_url.

Configuration Example

To use a custom endpoint (e.g., S3NS), one can now configure fog like this:

connection_options = {
  provider:					'Google',
  google_project:			'my-project',
  google_json_key_string:	'{"my": "json_creds"}',
  google_json_root_url:		'https://s3ns.example.com/storage/v1/', # Points to S3NS API
  host:						's3ns.example.com',
  scheme:					'https',
  port:						443
}