feat: token introspection (#83)

* introspect

* introspect and client assertion

* introspect and client assertion

* scopes

* token introspection

* introspect

* refactoring

* fixes

* clenaup

* Update example/internal/mock/storage.go

Co-authored-by: Fabi <38692350+fgerschwiler@users.noreply.github.com>

* clenaup

Co-authored-by: Fabi <38692350+fgerschwiler@users.noreply.github.com>

pkg/client/key.go

@@ -0,0 +1,40 @@
+package client
+
+import (
+	"encoding/json"
+	"io/ioutil"
+)
+
+const (
+	serviceAccountKey = "serviceaccount"
+	applicationKey    = "application"
+)
+
+type keyFile struct {
+	Type   string `json:"type"` // serviceaccount or application
+	KeyID  string `json:"keyId"`
+	Key    string `json:"key"`
+	Issuer string `json:"issuer"` //not yet in file
+
+	//serviceaccount
+	UserID string `json:"userId"`
+
+	//application
+	ClientID string `json:"clientId"`
+}
+
+func ConfigFromKeyFile(path string) (*keyFile, error) {
+	data, err := ioutil.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	return ConfigFromKeyFileData(data)
+}
+
+func ConfigFromKeyFileData(data []byte) (*keyFile, error) {
+	var f keyFile
+	if err := json.Unmarshal(data, &f); err != nil {
+		return nil, err
+	}
+	return &f, nil
+}