Refactor client registration for draft-ietf-oauth-dyn-reg-12

[Vladimir Dzhuvinov]

Aims:

• Add support for client update and delete operations
• Split current ClientDetails into ClientMetadata and ClientRegistration extending it (or similar solution).

[Vladimir Dzhuvinov]

Refactoring complete for client registration and client read request / response. Client delete supported only as request. Due to the complex class hierarchy that needs to extend to the OIDC package we need to spend time considering the possible solution (to do later, after server alpha release).