Allow to export/import single configuration by language

Problem/Motivation

The single configuration export and import forms at admin/config/development/configuration/single/export and admin/config/development/configuration/single/import only work with the default configuration collection. When a site has multiple languages enabled with the Configuration Translation module, configuration translations are stored in non-default collections (e.g., language.fr, language.de, etc.).

Currently, there is no way for site administrators to export or import individual translated configuration items through the UI/drush.

Steps to reproduce

1. Install Drupal with the Configuration Translation and Language modules enabled
2. Add a second language (e.g., French) at admin/config/regional/language
3. Translate a configuration item (e.g., system.site) via the config translation interface
4. Run drush cex to export all configuration - notice that a language/fr/ directory is created with the translated config
5. Go to admin/config/development/configuration/single/export
6. Select "Simple configuration" and "system.site"

Expected result: There should be a way to select and export the French translation of system.site from the language.fr collection.

Actual result: Only the default collection config is shown. No option exists to select or view translated configuration items. The French translation at language/fr/system.site.yml is not accessible through this UI.

Proposed resolution

Add a "Collection" selector to both the single export and single import forms that allows administrators to choose which configuration collection to work with.

The collection selector should:

• Default to the "Default" collection (current behavior)
• List all available collections in the active configuration storage (e.g., "Default", "language.fr", "language.de", "language.es")
• When a non-default collection is selected, update the configuration item dropdown to show only items available in that collection