Metadata Repositories

In preparation of metadata pushing and to allow EngineBlock to work on metadata from a SAML2 Metadata Document (like EduGain).
* Abstract array based SP / IdP metadata into full entities.
* Introduce 2 repositories
    1. JanusRestV1, legacy REST / RPC API.
    2. Stoker, a new component, OpenConext Stoker can retrieve and efficiently synchronise between local file system and a SAML2 Metadata document, this repository can read from Stoker metadata.