Support Device Authorization Response

[sjohnr]

Add basic support for OAuth 2.0 Device Authorization Grant in oauth2-core, including:

• Parameter names in OAuth2ParameterNames
• Classes for device code and user code that extend AbstractOAuth2Token
• HttpMessageConverter for Device Authorization Response
• New AuthorizationGrantType constant for urn:ietf:params:oauth:grant-type:device_code

Issue spring-projects/spring-authorization-server#44