notification: add OnUserRolesUpdated (#58)

Adds the `UserRolesUpdated` notification event and handleReceive logic
Part of CORE-206
## Test plan
CI

notifications/v1/subscriber.go

@@ -180,6 +180,13 @@ type SubscriberHandlers struct {
 	// MUST make sure the error is surfaced (by either returning or logging the
 	// error) to be retried or to a human operator.
 	OnUserDeleted func(ctx context.Context, data *UserDeletedData) error
+	// OnUserRolesUpdated is called when a "UserRolesUpdated" notification is received.
+	//
+	// It indicates that a user's roles have been updated for a particular service.
+	// The notification data does not specify whether roles have been granted or revoked.
+	// If the service's roles are relevant to the subscriber the user's current roles can
+	// be retrieved from the SAMS API.
+	OnUserRolesUpdated func(ctx context.Context, data *UserRolesUpdatedData) error
 }
 
 type ReceiveSettings = pubsub.ReceiveSettings
@@ -201,6 +208,16 @@ func (s *subscriber) handleReceive(ctx context.Context, name string, metadata js
 		}
 
 		return "handled", s.handlers.OnUserDeleted(ctx, &data)
+	case nameUserRolesUpdated:
+		if s.handlers.OnUserRolesUpdated == nil {
+			return "skipped", nil
+		}
+		var data UserRolesUpdatedData
+		if err := json.Unmarshal(metadata, &data); err != nil {
+			return "malformed_message", errors.Wrap(err, "unmarshal metadata")
+		}
+
+		return "handled", s.handlers.OnUserRolesUpdated(ctx, &data)
 	}
 
 	// Unknown message type

notifications/v1/types.go

@@ -1,12 +1,16 @@
 package v1
 
-import "go.opentelemetry.io/otel"
+import (
+	"github.com/sourcegraph/sourcegraph-accounts-sdk-go/services"
+	"go.opentelemetry.io/otel"
+)
 
 // ⚠️ WARNING: These types MUST match the SAMS implementation, at
 // backend/internal/notification/types.go
 
 const (
-	nameUserDeleted = "UserDeleted"
+	nameUserDeleted      = "UserDeleted"
+	nameUserRolesUpdated = "UserRolesUpdated"
 )
 
 // UserDeletedData contains information of a "UserDeleted" notification.
@@ -17,4 +21,10 @@ type UserDeletedData struct {
 	Email string `json:"email"`
 }
 
+// UserRolesUpdatedData contains information of a "UserRolesUpdated" notification.
+type UserRolesUpdatedData struct {
+	AccountID string           `json:"account_id"`
+	Service   services.Service `json:"service"`
+}
+
 var tracer = otel.Tracer("sams.notifications.v1")