Create a `class HttpxFetcher(FetcherInterface)` for `httpx`

[KOLANICH]

Description of issue or feature request:
requests doesn't support HTTP/2

Current behavior:
no HttpxFetcher

Expected behavior:
HttpxFetcher is implemented

[jku]

A reasonable request. I'll just note that it is possible to implement this outside of python-tuf as well: this is why FetcherInterface is public API.