Add HTTP backend

[kreczko]

This PR adds a new backend for HTTP.
The idea is that a web service can provide the checksum retrieval as long as it is of the form

@router.get("/checksum/{checksum_type}")
def checksum(path: str, checksum_type: str) -> PlainTextResponse:

--→ the path is taken as a parameter and the checksum type part of the URL.
Server URL and base path can be configured via the HttpSettings.