Add support for the `trust` command

[gesellix]

Along with the trust command (as of Docker 17.10.x), there should be full fledged support for general handling of trusted images using Notary.

We'll probably need a Java client for TUF (https://theupdateframework.github.io/)? 🤔

For inspirations see

• docker/cli/e2e/compose-env.yaml
• docker/cli/trust
• docker/cli/command/image/trust.go
• docker/cli/command/image/pull.go