Mp4: Ability to add custom user tags per video (like AWS)

[mraduldubey]

Is your feature request related to a problem? Please describe.
Ability to add multiple custom user tags per video. Just like multiple user defined tags (key-value pairs) can be created for any resource on AWS. This enables saving any custom metadata for the videos for eg. custom video verification/authentication algo

Describe the solution you'd like
Update libmp4 to add user metadata - which is basically as table for user defined key value pairs.
Expose the functionality in mp4 writer and reader

Describe alternatives you've considered
Hijacking the ISO defined tags in the mp4 headers. But they are limited and lack scalability

Additional context
Smart Video content authentication