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/...`

internal/testutil/gen/gen.go

@@ -0,0 +1,58 @@
+// Package gen allows generating of example tokens and claims.
+//
+//	go run ./internal/testutil/gen
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+
+	tu "github.com/zitadel/oidc/v2/internal/testutil"
+	"github.com/zitadel/oidc/v2/pkg/oidc"
+)
+
+var custom = map[string]any{
+	"foo": "Hello, World!",
+	"bar": struct {
+		Count int      `json:"count,omitempty"`
+		Tags  []string `json:"tags,omitempty"`
+	}{
+		Count: 22,
+		Tags:  []string{"some", "tags"},
+	},
+}
+
+func main() {
+	enc := json.NewEncoder(os.Stdout)
+	enc.SetIndent("", "    ")
+
+	accessToken, atClaims := tu.NewAccessTokenCustom(
+		tu.ValidIssuer, tu.ValidSubject, tu.ValidAudience,
+		tu.ValidExpiration.AddDate(99, 0, 0), tu.ValidJWTID,
+		tu.ValidClientID, tu.ValidSkew, custom,
+	)
+	atHash, err := oidc.ClaimHash(accessToken, tu.SignatureAlgorithm)
+	if err != nil {
+		panic(err)
+	}
+
+	idToken, idClaims := tu.NewIDTokenCustom(
+		tu.ValidIssuer, tu.ValidSubject, tu.ValidAudience,
+		tu.ValidExpiration.AddDate(99, 0, 0), tu.ValidAuthTime,
+		tu.ValidNonce, tu.ValidACR, tu.ValidAMR, tu.ValidClientID,
+		tu.ValidSkew, atHash, custom,
+	)
+
+	fmt.Println("access token claims:")
+	if err := enc.Encode(atClaims); err != nil {
+		panic(err)
+	}
+	fmt.Printf("access token:\n%s\n", accessToken)
+
+	fmt.Println("ID token claims:")
+	if err := enc.Encode(idClaims); err != nil {
+		panic(err)
+	}
+	fmt.Printf("ID token:\n%s\n", idToken)
+}