Add support for externally hosted WebGL content (no ZIP upload)

[hari715om]

Summary

This PR adds support for embedding externally hosted Unity WebGL builds in the WebGL activity without requiring ZIP file uploads.

Teachers can now provide a public index.html URL for large WebGL builds that are already hosted on a web server or CDN.

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Motivation

Uploading Unity WebGL ZIP archives (often 400MB+) through Moodle is slow, error-prone, and unnecessary when content is already hosted externally.

This enhancement allows:

• Faster activity creation
• Reduced server load
• Better scalability for large WebGL projects

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Key Changes

• Added a new storage option: External WebGL URL
• Added URL input field for externally hosted index.html
• ZIP upload is conditionally required only for existing storage engines
• Existing Azure, S3, and Moodle File API workflows are unchanged

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Backward Compatibility

• No changes to existing database schema
• All existing storage engines continue to require ZIP uploads
• External URL mode bypasses ZIP processing safely