feat: add support for attachment collections on a model

[mauricewijnia]

This pull request introduces support for organizing attachments into named collections, making it easier to group and query related attachments. The changes add a new collection column to the attachables table, update the documentation to show how to use collections, and provide a convenient relationship method for querying attachments by collection.

Database migration and model enhancements:

• Added a new migration file add_collection_to_attachables_table.php.stub to introduce a nullable collection column to the attachables table, enabling attachments to be grouped by collection.
• Registered the new migration in LaravelAttachmentLibraryServiceProvider.php to ensure it runs with the package's migrations.
• Added the attachmentCollection method to the HasAttachments trait, allowing easy querying of attachments belonging to a specific collection.

Documentation updates:

• Updated usage documentation to show how to attach an attachment to a specific collection and how to retrieve attachments by collection using the new relationship method. [1] [2]

[dnwjn]

GREAT improvement! 🚀