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 allow setting the application's destination to a cluster, reducing the operational steps and enabling users to seamlessly use Kurator in a single-cluster, thereby 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
+
+- Add a new feature to support setting the application's destination to a cluster.
+- Enable users to seamlessly use Kurator in a single-cluster environment, thus eliminating the need for Fleet.
+
+### Proposal
+
+Currently, the application registration is handled in the fleet-manager. The proposal is to add a new feature to deploy the application directly to the current host cluster when ApplicationDestination is not specified.
+
+The preliminary discussion results are as follows:
+
+- If `ApplicationDestination` is not specified, the `application` is deployed directly under the current host cluster, otherwise it is deployed in `ApplicationDestination.fleet`
+- When updating the destination configuration, no actions will be taken on applications deployed in the old clusters (no deletion).
+
+
+
+### Design Details
+
+This proposal does not involve API modification, and supports new functions in code logic.
+In the specific code, if `ApplicationDestination` is empty, `handleSyncPolicyByKind` directly on the current cluster client to create Kustomization or HelmRelease resources.
+
+
+#### 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.