Add optional setting to use system temp directory for H5P temporary files

[Lin87]

Hello,

This pull request introduces an optional admin setting that allows H5P to store temporary/working files in the server’s system temporary directory (via PHP's sys_get_temp_dir() function) instead of the H5P plugin directory.

Background

In our environment, WordPress content is stored on AWS Elastic File System (EFS). While EFS works well for persistent content, it performs poorly when handling large numbers of small, frequently changing files—such as those created during H5P uploads and processing.

By using the server's system temp directory for these temporary files, we've observed:

• Significantly faster upload and processing times
• Reduced I/O overhead on networked file systems like EFS
• Improved performance on resource-constrained servers

This aligns with common best practices of using local system temp storage for short-lived working files.

What This PR Does

• Adds a new admin-configurable setting to enable use of the system temp directory for H5P temporary files
• Keeps the setting disabled by default, preserving existing behavior
• Ensures the change is fully opt-in and does not impact existing installations unless explicitly enabled by an administrator

Backward Compatibility & Safety

• No changes to default behavior
• No migration required
• Existing installs will continue to use the current plugin directory unless the setting is enabled
• The feature is isolated to temporary file handling only

This pull request is based on the earlier work from PR #130, originally submitted several years ago. That PR was created by my colleague and did not receive any follow-up, so this submission reintroduces the feature with minor updates to align with the current H5P codebase.

Thank you for considering this improvement. I'm happy to adjust the implementation or documentation if needed.