GODRIVER-3565

Summary

Update GridFSFile to implement bson.Unmarshaler

Background & Motivation

The gridfs.File object was initially added in GODRIVER-1548. At that time, the UnmarshalBSON method was implemented to allow documents from the "files" collection to decode into the object, while avoiding the use of explicit BSON tags. However, the original PR does not clearly explain why adding BSON tags was intentionally avoided.

Later, GODRIVER-2606 proposed deprecating the UnmarshalBSON method, citing that much of its functionality was unused in the Go Driver API and retained only for backward compatibility.

As part of the Go Driver 2.0 efforts, UnmarshalBSON was removed in GODRIVER-2617.

This change introduces an issue with the GridFSBucket.Find() method, which returns a cursor streaming documents that map directly to the GridFSFile type. As a result, users can no longer decode this stream directly into instances of GridFSFile`, as shown below:

for fileCursor.Next(context.Background()) {
	var file GridFSFile
	err := fileCursor.Decode(&file)
	require.NoError(t, err)

	assert.NotNil(t, file.ID) // Fails
}

GODRIVER-3565 suggests adding UnmarshalBSON back to GridFSFile as a convenience to users.