Add entities descriptor (#54)

* Add `EntitiesDescriptor`

* Add `EntityDescriptorType` enum to allow to parse arbitrarily nested `EntityDescriptor`s and `EntitiesDescriptor`s

* Update CHANGELOG

---------

Co-authored-by: Daniel Wiesenberg <[email protected]>
Co-authored-by: Nathan Jaremko <[email protected]>

Author: Weasy666

CHANGELOG

@@ -1,13 +1,13 @@
 # Changelog
 
-
 ### Unreleased
 
 ### 0.0.16
 
 - Add support for basic `RequestedAuthnContext` de-/serialization in `AuthnRequest`
 - Add support for Elliptic-curve cryptography
 - Use enum for signature and digest algorithm
+- Add EntityDescriptorType enum to allow parsing of arbitrarily nested EntityDescriptors and EntitiesDescriptors
 
 ### 0.0.15