feat: add optional ?server=local/pre_ckan query param to PUT /url/{resource_id}

[rbardaji]

Overview
This pull request adds an optional server query parameter to the
PUT /url/{resource_id} endpoint, allowing users to specify whether they
want to update the resource in the local CKAN instance or in the
pre-CKAN instance. If no server parameter is provided, the endpoint
defaults to 'local' to maintain backward compatibility.

Changes

1. Introduced a ckan_instance parameter in update_url to avoid
   relying solely on ckan_settings.ckan.
2. Added ?server=local or ?server=pre_ckan to the PUT /url/{resource_id}
   route.
3. Returns a 400 error if pre_ckan is disabled (pre_ckan_enabled=False)
   or if the pre_ckan URL is missing a valid scheme ("No scheme supplied").
4. Preserves existing behavior when the server parameter is not specified,
   defaulting to local CKAN.

Impact

• Provides explicit control over which CKAN instance to use for URL
  resource updates.
• Delivers user-friendly error messages if the pre_ckan configuration
  is invalid.
• No breaking changes for existing clients that do not pass ?server=.

Testing

• Verified locally that ?server=pre_ckan uses the pre-CKAN instance
  if enabled and properly configured.
• Confirmed that calls without ?server continue using local CKAN
  without any change in behavior.