Implement `tuf regenerate`

[lmars]

I should be able to run tuf regenerate to regenerate targets.json based on the target files in the committed targets directory.

[mnm678]

There is a skeleton of this here: https://github.com/theupdateframework/go-tuf/blob/master/cmd/tuf/regenerate.go

[rdimitrov]

Closing since the code base changed and the goal of the project would be to focus on being a library first and not so much on providing a CLI client.

There might be a CLI based on this, but it will live in a separate repository.

Thanks for raising this 👍