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

pkg/op/verifier_access_token.go

@@ -68,28 +68,28 @@ func NewAccessTokenVerifier(issuer string, keySet oidc.KeySet, opts ...AccessTok
 }
 
 // VerifyAccessToken validates the access token (issuer, signature and expiration)
-func VerifyAccessToken(ctx context.Context, token string, v AccessTokenVerifier) (oidc.AccessTokenClaims, error) {
-	claims := oidc.EmptyAccessTokenClaims()
+func VerifyAccessToken[C oidc.Claims](ctx context.Context, token string, v AccessTokenVerifier) (claims C, err error) {
+	var nilClaims C
 
 	decrypted, err := oidc.DecryptToken(token)
 	if err != nil {
-		return nil, err
+		return nilClaims, err
 	}
-	payload, err := oidc.ParseToken(decrypted, claims)
+	payload, err := oidc.ParseToken(decrypted, &claims)
 	if err != nil {
-		return nil, err
+		return nilClaims, err
 	}
 
 	if err := oidc.CheckIssuer(claims, v.Issuer()); err != nil {
-		return nil, err
+		return nilClaims, err
 	}
 
 	if err = oidc.CheckSignature(ctx, decrypted, payload, claims, v.SupportedSignAlgs(), v.KeySet()); err != nil {
-		return nil, err
+		return nilClaims, err
 	}
 
 	if err = oidc.CheckExpiration(claims, v.Offset()); err != nil {
-		return nil, err
+		return nilClaims, err
 	}
 
 	return claims, nil