feat(coderd): add mark-all-as-read endpoint for inbox notifications (#16976)

[Resolve this issue](https://github.com/coder/internal/issues/506)

Add a mark-all-as-read endpoint which is marking as read all
notifications that are not read for the authenticated user.
Also adds the DB logic.

coderd/apidoc/docs.go

@@ -1705,6 +1705,25 @@ const docTemplate = `{
                 }
             }
         },
+        "/notifications/inbox/mark-all-as-read": {
+            "put": {
+                "security": [
+                    {
+                        "CoderSessionToken": []
+                    }
+                ],
+                "tags": [
+                    "Notifications"
+                ],
+                "summary": "Mark all unread notifications as read",
+                "operationId": "mark-all-unread-notifications-as-read",
+                "responses": {
+                    "204": {
+                        "description": "No Content"
+                    }
+                }
+            }
+        },
         "/notifications/inbox/watch": {
             "get": {
                 "security": [

coderd/apidoc/swagger.json

@@ -1486,6 +1486,23 @@
 				}
 			}
 		},
+		"/notifications/inbox/mark-all-as-read": {
+			"put": {
+				"security": [
+					{
+						"CoderSessionToken": []
+					}
+				],
+				"tags": ["Notifications"],
+				"summary": "Mark all unread notifications as read",
+				"operationId": "mark-all-unread-notifications-as-read",
+				"responses": {
+					"204": {
+						"description": "No Content"
+					}
+				}
+			}
+		},
 		"/notifications/inbox/watch": {
 			"get": {
 				"security": [