Add missing setters and getters for the services configuration properties.

[EddyTheCo]

Many of the service Configuration properties in https://git.kernel.org/pub/scm/network/connman/connman.git/tree/doc/service-api.txt are still not implemented.
The latter produces warnings when parsing the service properties:

Unknown or empty property for Service: IPv4.Configuration
Unknown or empty property for Service: IPv6.Configuration
Unknown or empty property for Service: Timeservers.Configuration
Unknown or empty property for Service: Domains.Configuration
Unknown or empty property for Service: Proxy.Configuration
Unknown or empty property for Service: mDNS.Configuration

and limits the user's ability to change configurations using this library.

Setting and getting the Configuration properties has already been done for Nameservers.Configuration

libcppconnman/include/amarula/dbus/connman/gservice.hpp

Line 256 in f63926c

void setNameServers(const std::vector<std::string>& name_servers,

One can use this as a reference.