the MD5 scheme should still be widely deployed, even though it is
deprecated. it is also still the default if not specific algorithm
is requested by the server.
we previously performed a whole new GET request when doing digest
authentication. it seemed beneficial to reuse the TCP connection to
perform the second GET request, which includes the authentication
tokens. however, if the server sends "Connection: close" we must not
requse the TCP connection for another HTTP request.
this broke authentication against Shelly devices (at least those with
original firmware).
now we explicitly set "Connection: keep-alive" in our request, and reuse
the TCP connection only if te server replies with "Connection:
keep-alive" as well.
this new class uses the newly introduced HttpRequestConfig and performs
HTTP requests using this config. it will be reused for other power
meters (SML over HTTP(S)) and may be reused by other features in the
future (battery provider, solar power provider, etc.).
