Add inflection MDRPI

Utilised for all models that support SAML V2.0 Metadata Extensions for
Registration and Publication Information.
Add mdrpi:RegistrationInfo model
Add mdrpi:RegistrationPolicy model
Add mdrpi:PublicationInfo model
Add mdrpi:UsagePolicy model
Extend factories to support MDRPI

Allow both EntitiesDescriptors and EntityDescriptors to generate
extended MDRPI related data.