Refactor to find components for tuf itself

[jku]

There may be components here that could be integrated into the python tuf library, and changes in TUF that would help this tool. Some ideas:

• Try to separate individual Metadata editing functionality from the rest (the parts that do not require knowledge of the rest of the repository)
• the way ngclient internals are used is not ideal:
  • maybe TrustedMetadataSet should be more public?
  • should tufrepo re-implement the updater metadata loading (so it didn't need to do the tricks with Filesystemfetcher and then modifying the internal TrustedMetadataSet)
• tuf.api.metadata.Targets should have a add_key() method
• Modifying tuf.api.metadata.Targets.delegations is hard to do correctly currently

[jku]

private keys are no longer parsed in repo.py

Potential future work:

• Don't read/write metadata files repo.py (not sure how realistic this is but otherwise repo.py can't be a generic repo editor...)
• metadata loading should be re-implemented (and Updater should not be used)
• TrustedMetadataSet maybe should be public and have more granularity in error situations

[jku]

Possibly the git integration could be removed from the Repo class -- and handled in the CLI instead?