Refactor authorization response URL building

Adds an implicit grant response version and refactors shared code into
a separate buildURL function.
Implement id_token creation for openid requests

Adds the id_token to token endpoint responses for openid. This is still
a stop-gap approach using defaults (JWS RS256) until proper storage of
client metadata is implemented.
Support id_token response_type

Adds creation of id_token for implicit grant in authorization
request handling.