Introducing public field to client models

[jarlandre]

• denying client credentials grant if client is public
• adjusting tests to make different clients based on whether or not they are public
• this will allow for using public grants like authorisation code flow with client_secret not set on client and Public set to true
• if libraries implement this and dont want to care about it they can just return false for IsPublic (i forked go-oauth2-pg and changed it to contain public field in db)