Add custom metadata in MP4 such as fingerprint for authenticity

[cedricve]

Hey there! Thanks for the great work. We have been working on https://github.com/kerberos-io/agent for a few years now, and we have been successfully integrated your library in our core product which is now used by thousands of users (open source). https://github.com/kerberos-io/agent/blob/master/machinery/src/capture/main.go#L228-L242

We are looking into adding custom metadata, such as an encrypted fingerprint, so the authenticity of the MP4 can be validated (in the event of altering) or "faking" the fingerprint, we could validate be decrypting using a public key.

To achieve this we were wondering what's the best way to write this metadata into the MP4 structure, as we could not see how this could be done with the current implementation. Happy to brainstorm.

I thought to have something in the WriteTrailer method that would close the MP4 and add some metadata on top (hash of the entire MP4 for example + encrypted).

https://github.com/kerberos-io/agent/blob/master/machinery/src/capture/main.go#L289-L291