Add support for SDPi Discovery Proxy

[DominikST95]

In order to test applications or devices that use the Discovery Proxy, add a configuration entry for Discovery Proxy address and use it for Discovery purposes.

As specification, use the SDPi specification implemented here: https://gitlab.com/sdc-suite/dpws-discovery-proxy
(Specification link still pending)

[ldeichmann]

A spec appears to be available in SDPi now, see https://profiles.ihe.net/DEV/SDPi/index.html#vol1_clause_sdpi_p_option_discovery_proxy or https://profiles.ihe.net/DEV/SDPi/index.html#vol2_clause_appendix_mdpws_discovery_proxy or many other places in the document