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/oidc/token_request.go

@@ -187,12 +187,12 @@ func (j *JWTTokenRequest) GetAudience() []string {
 
 // GetExpiration implements the Claims interface
 func (j *JWTTokenRequest) GetExpiration() time.Time {
-	return time.Time(j.ExpiresAt)
+	return j.ExpiresAt.AsTime()
 }
 
 // GetIssuedAt implements the Claims interface
 func (j *JWTTokenRequest) GetIssuedAt() time.Time {
-	return time.Time(j.IssuedAt)
+	return j.ExpiresAt.AsTime()
 }
 
 // GetNonce implements the Claims interface