Add normalize callback to pre-process files before processing

[timfjord]

Introduces an optional normalize/1 callback to allow transforming/normalising the file after validation and before version processing/storage.
This enables use-cases like renaming files(generating unique file names) or fixing metadata in a centralised, explicit step.

The idea of this PR was born when I was looking for a way to generate unique file names for uploaded files.
There is the filename/2 function, but it is for runtime filename calculation and heavily relies on the scope, and I was looking for something that would rename the file before even processing the versions.

[achempion]

Would it be possible to pass the renamed File struct to the Uploader.store/1?

upload |> File.new() |> Map.put(:file_name, "normalized") |> then(&Uploader.store({&1, current_user}))

[timfjord]

As per the current implementation, after normalisation, we pass whatever the normalize/1 returns to the put_versions/2 (partly, the goal of this extra step was to modify the File struct before creating versions).

So if I understand the question correctly, we do pass the renamed File struct

[achempion]

@timfjord I meant, can you just call the normalize function before calling waffle? ex: User uploads a file and you can call the normalize to change the file name before passing the file struct to Waffle

[timfjord]

Yeah, it is possible, but it makes the overall experience a bit inconsistent because there is no way to guarantee that the final file will have a unique name.
For example, in tests with something like https://github.com/beam-community/ex_machina in place, you need to duplicate this logic for every factory where a Waffle field is involved.
The same goes for some mix scripts, etc.
So basically every time you interact with a Waffle field, you need to remember to normalise whatever you pass there.

That's why it would be nice to have this normalisation option built into Waffle.