example: add grpc example

examples/config-grpc.yaml

+issuer: http://127.0.0.1:5556
+storage:
+  type: sqlite3
+  config:
+    file: examples/dex.db
+
+web:
+  http: 127.0.0.1:5556
+
+grpc:
+  addr: 127.0.0.1:5557
+
+connectors:
+- type: mockCallback
+  id: mock-callback
+  name: Mock
+- type: mockPassword
+  id: mock-password
+  name: Password
+  config:
+    username: "admin"
+    password: "PASSWORD"
+
+staticClients:
+- id: example-app
+  redirectURIs:
+  - 'http://127.0.0.1:5555/callback'
+  name: 'Example App'
+  secret: ZXhhbXBsZS1hcHAtc2VjcmV0