[6.x] Make Files fieldtype temporary upload storage configurable

[duncanmcclean]

This pull request makes the Files fieldtype's temporary upload storage configurable, allowing files to be stored on a shared filesystem for multi-server environments.

Previously, temporary file uploads were hardcoded to use the local disk at statamic/file-uploads. This caused issues in multi-server setups where a file might be uploaded on one server but processed (e.g., queued form emails with attachments) on another.

This PR adds two new config options in config/system.php:

'file_uploads' => [
    'disk' => env('STATAMIC_FILE_UPLOADS_DISK', 'local'),
    'path' => env('STATAMIC_FILE_UPLOADS_PATH', 'statamic/file-uploads'),
],

These can be configured via environment variables to use a shared filesystem like S3:

STATAMIC_FILE_UPLOADS_DISK=s3
STATAMIC_FILE_UPLOADS_PATH=statamic/file-uploads

This PR also adds a test to verify the configured disk is used.

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Requested on Twitter: https://x.com/adampatterson/status/2057113369623298060

[adampatterson]

Awesome, thanks so much!

This should work nicely with Livewire Forms