ngclient: Add new client code

This is a new client library implementation using the Metadata API
(the only "old" file used is exceptions.py)
* Functional but largely still untested
* Requires work before it can replace tuf.client but should be good
  enough to include in the repo

The major changes compared to current client so far are:
 * Use of Metadata API
 * Major simplification in the way downloads are handled due to removing
   mirrors support
 * Separating tracking of valid metadata into a separate component
 * There's no MultiRepoUpdater

We do not expect other major changes (in the sense of moving large
amounts of code) but do plan to possibly improve the client API. The
API has already changed so this is not going to be a 1:1 compatible
implementation, but porting should not be difficult.

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

tuf/ngclient/README.md

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+## Next-gen TUF client for Python
+
+This package provides modules for TUF client implementers.
+
+**tuf.ngclient.Updater** is a class that implements the client workflow
+described in the TUF specification (see
+https://theupdateframework.github.io/specification/latest/#detailed-client-workflow)
+
+**tuf.ngclient.FetcherInterface** is an abstract class that client
+implementers can optionally use to integrate with their own
+network/download infrastructure -- a Requests-based implementation is
+used by default.
+
+This package:
+* Aims to be a clean, easy-to-validate reference client implementation
+  written in modern Python
+* At the same time aims to be the library choice for anyone
+  implementing a TUF client in Python: light-weight, easy to integrate
+  and with minimal required dependencies
+* Is still under development but planned to become the default client
+  in this code base (as in the older tuf.client will be deprecated in
+  the future)

tuf/ngclient/__init__.py

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+# Copyright New York University and the TUF contributors
+# SPDX-License-Identifier: MIT OR Apache-2.0
+
+"""TUF client public API
+"""
+
+from tuf.ngclient.fetcher import FetcherInterface
+from tuf.ngclient.updater import Updater