Factorize IFile interface

[lemunozm]

Thread: https://kflabs.slack.com/archives/C07PG2EUR9C/p1748512638265439

We can generalize the file() method and File event to have the param encoded as follows:

file(bytes32 what, bytes callback value)

This would allow us to extract the method signature, event, and error into just one interface. Also, the indexer/event notifier only needs to track one administration event to warn about bad usages.