Standard implementation of ISigneeProvider interface

An implementation of the [ISingeeProvider](https://github.com/Altinn/app-lib-dotnet/issues/788) interface which reads from a configuration file the signee details.

This works well with Studio integration as the app developer would choose a the default implementation and in the gui point to where in the data model the relevant details are (Pid and Surname).

Start by reading issue 788 and the ISigneeProvider interface referenced there. Trace how implementations are selected and how configuration is supplied to an Altinn App, then define what the Studio integration must expose for mapping Pid and Surname. Done means a standard configuration-backed implementation can be selected as the default and reads the configured signee details.

Written by the indexing model from the issue text.