feat: Token Revocation, Request Object and OP Certification (#130)

FEATURES (and FIXES):
- support OAuth 2.0 Token Revocation [RFC 7009](https://datatracker.ietf.org/doc/html/rfc7009)
- handle request object using `request` parameter [OIDC Core 1.0 Request Object](https://openid.net/specs/openid-connect-core-1_0.html#RequestObject)
- handle response mode
- added some information to the discovery endpoint:
  - revocation_endpoint (added with token revocation) 
  - revocation_endpoint_auth_methods_supported (added with token revocation)
  - revocation_endpoint_auth_signing_alg_values_supported (added with token revocation)
  - token_endpoint_auth_signing_alg_values_supported (was missing)
  - introspection_endpoint_auth_signing_alg_values_supported (was missing)
  - request_object_signing_alg_values_supported (added with request object)
  - request_parameter_supported (added with request object)
 - fixed `removeUserinfoScopes ` now returns the scopes without "userinfo" scopes (profile, email, phone, addedd) [source diff](https://github.com/caos/oidc/pull/130/files#diff-fad50c8c0f065d4dbc49d6c6a38f09c992c8f5d651a479ba00e31b500543559eL170-R171)
- improved error handling (pkg/oidc/error.go) and fixed some wrong OAuth errors (e.g. `invalid_grant` instead of `invalid_request`)
- improved MarshalJSON and added MarshalJSONWithStatus
- removed deprecated PEM decryption from `BytesToPrivateKey`  [source diff](https://github.com/caos/oidc/pull/130/files#diff-fe246e428e399ccff599627c71764de51387b60b4df84c67de3febd0954e859bL11-L19)
- NewAccessTokenVerifier now uses correct (internal) `accessTokenVerifier` [source diff](https://github.com/caos/oidc/pull/130/files#diff-3a01c7500ead8f35448456ef231c7c22f8d291710936cac91de5edeef52ffc72L52-R52)

BREAKING CHANGE:
- move functions from `utils` package into separate packages
- added various methods to the (OP) `Configuration` interface [source diff](https://github.com/caos/oidc/pull/130/files#diff-2538e0dfc772fdc37f057aecd6fcc2943f516c24e8be794cce0e368a26d20a82R19-R32)
- added revocationEndpoint to `WithCustomEndpoints ` [source diff](https://github.com/caos/oidc/pull/130/files#diff-19ae13a743eb7cebbb96492798b1bec556673eb6236b1387e38d722900bae1c3L355-R391)
- remove unnecessary context parameter from JWTProfileExchange [source diff](https://github.com/caos/oidc/pull/130/files#diff-4ed8f6affa4a9631fa8a034b3d5752fbb6a819107141aae00029014e950f7b4cL14)

pkg/op/keys_test.go

@@ -0,0 +1,100 @@
+package op_test
+
+import (
+	"crypto/rsa"
+	"math/big"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/golang/mock/gomock"
+	"github.com/stretchr/testify/assert"
+	"gopkg.in/square/go-jose.v2"
+
+	"github.com/caos/oidc/pkg/oidc"
+	"github.com/caos/oidc/pkg/op"
+	"github.com/caos/oidc/pkg/op/mock"
+)
+
+func TestKeys(t *testing.T) {
+	type args struct {
+		k op.KeyProvider
+	}
+	type res struct {
+		statusCode  int
+		contentType string
+		body        string
+	}
+	tests := []struct {
+		name string
+		args args
+		res  res
+	}{
+		{
+			name: "error",
+			args: args{
+				k: func() op.KeyProvider {
+					m := mock.NewMockKeyProvider(gomock.NewController(t))
+					m.EXPECT().GetKeySet(gomock.Any()).Return(nil, oidc.ErrServerError())
+					return m
+				}(),
+			},
+			res: res{
+				statusCode:  http.StatusInternalServerError,
+				contentType: "application/json",
+				body: `{"error":"server_error"}
+`,
+			},
+		},
+		{
+			name: "empty list",
+			args: args{
+				k: func() op.KeyProvider {
+					m := mock.NewMockKeyProvider(gomock.NewController(t))
+					m.EXPECT().GetKeySet(gomock.Any()).Return(nil, nil)
+					return m
+				}(),
+			},
+			res: res{
+				statusCode:  http.StatusOK,
+				contentType: "application/json",
+			},
+		},
+		{
+			name: "list",
+			args: args{
+				k: func() op.KeyProvider {
+					m := mock.NewMockKeyProvider(gomock.NewController(t))
+					m.EXPECT().GetKeySet(gomock.Any()).Return(
+						&jose.JSONWebKeySet{Keys: []jose.JSONWebKey{
+							{
+								Key: &rsa.PublicKey{
+									N: big.NewInt(1),
+									E: 1,
+								},
+								KeyID: "id",
+							},
+						}},
+						nil,
+					)
+					return m
+				}(),
+			},
+			res: res{
+				statusCode:  http.StatusOK,
+				contentType: "application/json",
+				body: `{"keys":[{"kty":"RSA","kid":"id","n":"AQ","e":"AQ"}]}
+`,
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			w := httptest.NewRecorder()
+			op.Keys(w, httptest.NewRequest("GET", "/keys", nil), tt.args.k)
+			assert.Equal(t, tt.res.statusCode, w.Result().StatusCode)
+			assert.Equal(t, tt.res.contentType, w.Header().Get("content-type"))
+			assert.Equal(t, tt.res.body, w.Body.String())
+		})
+	}
+}