feat(coderd): add inbox notifications endpoints (#16889)

This PR is part of the inbox notifications topic, and rely on previous
PRs merged - it adds :

- Endpoints to : 
  - WS : watch new inbox notifications
  - REST : list inbox notifications
  - REST : update the read status of a notification

Also, this PR acts as a follow-up PR from previous work and : 

- fix DB query issues
- fix DBMem logic to match DB

coderd/pubsub/inboxnotification.go

@@ -0,0 +1,43 @@
+package pubsub
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/google/uuid"
+	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/codersdk"
+)
+
+func InboxNotificationForOwnerEventChannel(ownerID uuid.UUID) string {
+	return fmt.Sprintf("inbox_notification:owner:%s", ownerID)
+}
+
+func HandleInboxNotificationEvent(cb func(ctx context.Context, payload InboxNotificationEvent, err error)) func(ctx context.Context, message []byte, err error) {
+	return func(ctx context.Context, message []byte, err error) {
+		if err != nil {
+			cb(ctx, InboxNotificationEvent{}, xerrors.Errorf("inbox notification event pubsub: %w", err))
+			return
+		}
+		var payload InboxNotificationEvent
+		if err := json.Unmarshal(message, &payload); err != nil {
+			cb(ctx, InboxNotificationEvent{}, xerrors.Errorf("unmarshal inbox notification event"))
+			return
+		}
+
+		cb(ctx, payload, err)
+	}
+}
+
+type InboxNotificationEvent struct {
+	Kind              InboxNotificationEventKind `json:"kind"`
+	InboxNotification codersdk.InboxNotification `json:"inbox_notification"`
+}
+
+type InboxNotificationEventKind string
+
+const (
+	InboxNotificationEventKindNew InboxNotificationEventKind = "new"
+)