[backend] Add a new file provider to reference external files

[lecafard]

This commit introduces a new "external" file provider, allowing challenges to reference files hosted on external URLs. This is useful for large files or files that are already hosted elsewhere.

To support this, the file service has been refactored:

• The FileProviderInstance interface has been updated. The upload method now returns an object containing the ref, hash, and size of the file. This responsibility was previously handled by the FileService itself.
• The local and s3 providers have been updated to implement the new interface, using a new summarizeFile utility to calculate the hash and size from the upload stream.
• A new external provider is added. Instead of a file, it accepts a small JSON payload containing the url, hash, and size.
  • The external provider is instantiated directly and cannot be removed as we don't really want users to instantiate it.
• The admin file upload endpoint now accepts a provider query parameter to select which provider to use.
• There is a backwards incompatible change with the file hash since it's definitely easier if it's represented as a string.

see #273

[mcdcam]

I like this design. Uploading a JSON blob feels a bit hacky, but I haven't been able to think of an alternative that doesn't compromise somewhere else.

I think it'd be good to add some comments explaining that the uploaded "file" is just arbitrary data to be processed by the file provider.