refactor: use struct types for claim related types (#283)

* oidc: add regression tests for token claim json

this helps to verify that the same JSON is produced,
after these types are refactored.

* 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

- Changed UserInfoAddress to pointer in UserInfo and
IntrospectionResponse.
This was needed to make omitempty work correctly.
- Copy or merge maps in IntrospectionResponse and SetUserInfo

* op: add example for VerifyAccessToken

* fix: rp: wrong assignment in WithIssuedAtMaxAge

WithIssuedAtMaxAge assigned its value to v.maxAge, which was wrong.
This change fixes that by assiging the duration to v.maxAgeIAT.

* rp: add VerifyTokens example

* oidc: add standard references to:

- IDTokenClaims
- IntrospectionResponse
- UserInfo

* only count coverage for `./pkg/...`

pkg/client/rs/resource_server.go

@@ -112,7 +112,7 @@ func WithStaticEndpoints(tokenURL, introspectURL string) Option {
 	}
 }
 
-func Introspect(ctx context.Context, rp ResourceServer, token string) (oidc.IntrospectionResponse, error) {
+func Introspect(ctx context.Context, rp ResourceServer, token string) (*oidc.IntrospectionResponse, error) {
 	authFn, err := rp.AuthFn()
 	if err != nil {
 		return nil, err
@@ -121,7 +121,7 @@ func Introspect(ctx context.Context, rp ResourceServer, token string) (oidc.Intr
 	if err != nil {
 		return nil, err
 	}
-	resp := oidc.NewIntrospectionResponse()
+	resp := new(oidc.IntrospectionResponse)
 	if err := httphelper.HttpRequest(rp.HttpClient(), req, resp); err != nil {
 		return nil, err
 	}