proposal:Single cluster deployment application

docs/proposals/application/single_cluster_deployment_application.md

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+---
+title: Single cluster deployment application
+authors:
+- @yeyeye2333
+reviewers:
+approvers:
+
+creation-date: 2024-07-24
+
+---
+
+## Single Cluster Deployment Application
+
+### Summary
+
+This proposal aims to modify the application API to allow setting the application's destination to a cluster, reducing the operational steps required for Kurator in a single-cluster environment and increasing Kurator's flexibility.
+
+### Motivation
+
+Kurator leverages Fleet for multi-cluster management. However, this has resulted in the current application deployment being tightly coupled with Fleet, even requiring Fleet creation for single-cluster usage. Therefore, we aim to decouple the single-cluster client from Fleet.
+
+#### Goals
+
+- Modify the application API to support setting the application's destination to a cluster.
+- Simplify the deployment process for single-cluster environments by eliminating the need for Fleet.
+
+### Proposal
+
+Currently, the application registration is handled in the fleet-manager. The proposal is to move the application registration to the cluster-operator to support setting the application's destination to a cluster.
+
+### Design Details
+
+We will delve into the API design required to support setting the application's destination to a cluster. The following are the specific changes to the application:
+
+```go
+// ApplicationDestination defines the configuration to dispatch an artifact to a fleet or specific clusters.
+// Fleet and cluster are mutually exclusive.
+type ApplicationDestination struct {
+	// Fleet defines the fleet to dispatch the artifact.
+	// +optional
+	Fleet *FleetInfo `json:"fleetInfo,omitempty"`
+
+	// Cluster specifies a cluster to dispatch the artifact to.
+	// +optional
+	Cluster *corev1.ObjectReference `json:"clusters,omitempty"`
+}
+
+type FleetInfo struct {
+	// Name identifies the name of the fleet.
+	// +required
+	Name string `json:"name"`
+	// ClusterSelector specifies the selectors to select the clusters within the fleet.
+	// If unspecified, all clusters in the fleet will be selected.
+	// +optional
+	ClusterSelector *ClusterSelector `json:"clusterSelector,omitempty"`
+}
+```
+
+The preliminary discussion results are as follows:
+
+- The destination can only be either a fleet or a cluster. The `ApplicationWebhook.validate()` will check if only one of fleet or cluster is specified.
+- When updating the destination configuration, no actions will be taken on applications deployed in the old clusters (no deletion).
+
+In the specific code, replace all `fleet *fleetapi.Fleet` type parameters in the `Reconcile` method with `destinationObject interface{}`. In functions like `fetchClusterList` (previously `fetchFleetClusterList`) that need to parse `destinationObject`, use a `switch destinationObject.(type)` statement for different logical processing to improve code reusability.
+
+#### Test Plan
+
+During the coding phase, add unit tests covering core functionalities and edge cases. After completing the coding work, design integration tests to ensure the proper deployment of applications in a single cluster, and subsequently, provide examples of deploying applications in a single cluster.