refactor: use struct types for claim related types

BREAKING change.
The following types are changed from interface to struct type:

- AccessTokenClaims
- IDTokenClaims
- IntrospectionResponse
- UserInfo and related types.

The following methods of OPStorage now take a pointer to a struct type,
instead of an interface:

- SetUserinfoFromScopes
- SetUserinfoFromToken
- SetIntrospectionFromToken

The following functions are now generic, so that type-safe extension
of Claims is now possible:

- op.VerifyIDTokenHint
- op.VerifyAccessToken
- rp.VerifyTokens
- rp.VerifyIDToken

example/client/api/api.go

@@ -76,7 +76,7 @@ func main() {
 		params := mux.Vars(r)
 		requestedClaim := params["claim"]
 		requestedValue := params["value"]
-		value, ok := resp.GetClaim(requestedClaim).(string)
+		value, ok := resp.Claims[requestedClaim].(string)
 		if !ok || value == "" || value != requestedValue {
 			http.Error(w, "claim does not match", http.StatusForbidden)
 			return