implement RFC 8628: Device authorization grant

WIP

Related #264

pkg/oidc/device_authorization.go

@@ -0,0 +1,31 @@
+package oidc
+
+// DeviceAuthorizationRequest implements
+// https://www.rfc-editor.org/rfc/rfc8628#section-3.1,
+// 3.1 Device Authorization Request.
+type DeviceAuthorizationRequest struct {
+	Scopes   SpaceDelimitedArray `schema:"scope"`
+	ClientID string              `schema:"client_id"`
+}
+
+// DeviceAuthorizationResponse implements
+// https://www.rfc-editor.org/rfc/rfc8628#section-3.2
+// 3.2.  Device Authorization Response.
+type DeviceAuthorizationResponse struct {
+	DeviceCode              string `json:"device_code"`
+	UserCode                string `json:"user_code"`
+	VerificationURI         string `json:"verification_uri"`
+	VerificationURIComplete string `json:"verification_uri_complete,omitempty"`
+	ExpiresIn               int    `json:"expires_in"`
+	Interval                int    `json:"interval,omitempty"`
+}
+
+// DeviceAccessTokenRequest implements
+// https://www.rfc-editor.org/rfc/rfc8628#section-3.4,
+// Device Access Token Request.
+type DeviceAccessTokenRequest struct {
+	JWTTokenRequest
+	GrantType  string `json:"grant_type"`
+	DeviceCode string `json:"device_code"`
+	ClientID   string `json:"client_id"` // required, how??
+}