Feature request: GridFS provider support (MongoDB) for image sources

[klalexxx]

It would be very useful to add GridFS (MongoDB) as a supported image source/provider for imageproxy.

In many production setups, images are stored in MongoDB GridFS rather than on disk or in S3-compatible storage. Native GridFS support would allow imageproxy to be used directly as an image delivery and transformation layer without introducing an additional proxy or sync layer.

Related links

• MongoDB GridFS documentation: https://www.mongodb.com/docs/manual/core/gridfs/
• MongoDB Go Driver (GridFS): https://pkg.go.dev/go.mongodb.org/mongo-driver/mongo/gridfs

If this approach is acceptable, I am willing to fully implement the feature, including code, tests, and documentation.