Add support for delegating to a repository

[mnm678]

This is a draft that is missing support for

• [x] loading keys from a file
• [x] encrypting local key files
• [x] updating local TUF repository contents

For delegation support, this PR uses my pr'ed fork of go-tuf, which is available here if anyone would like to run this locally. This pr should not be merged until that pr is finalized.