docs: add kusion server roadmap

Author: ffforest

docs/design/kusion_server/roadmap.md

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+# Kusion Server Roadmap
+
+## Current gaps between upstream and internal version
+
+The following can be contributed back to upstream in August with the exception of `KCL Configuration Management`.
+
+### KCL Configuration Management
+- Initialize and modify `.k` files with new branches
+- Update configuration items
+- Manage configuration versions
+
+### AuthN, AuthZ
+- Support authentication based on JWT token
+  - Support auth whitelist
+  - Support RBAC if an IAM system is present
+  - Support ability to generate a token if an IAM system is present
+
+### Version management
+- Stack status management
+  - Persistent last `generated`/`previewed`/`applied` spec version
+  - Support explicit spec ID or content when `previewing`/`applying`
+
+### Resources management
+- Centrally write resource metadata and attributes after apply
+- Basic ability to look them up based on different criteria:
+  - Project/Stack/Workspace
+    - Example `"All resources for project X, stack Y in workspace Z"`
+  - ResourcePlane/ResourceType
+    - Example: `"All AWS S3 buckets for project X"`
+  - Any field in resource attributes
+    - Example: `"All MySQL databases in production with version 5.7 in region us-west-1"`
+
+### Secrets management
+- Manage application secrets with third-party vendors
+  - API to manage secrets associated with a `stack` (usually application secrets)
+  - Automatically replace them to actual values during preview/apply without showing them
+- Workspace credential management with third-party vendors
+  - API to manage `KubeConfig`/`AccessKey`/`Secretkey` associated with a `workspace`
+  - Automatically replace them to actual values during preview/apply without showing them
+
+### Quality-of-life improvements
+- Support applying through a federated cluster via a gateway runtime
+- The ability to list `projects`/`stacks`/`workspaces` with custom filters, for example:
+  - list all `projects` under org x
+  - list all `stacks` under org x and project y
+  - find `workspace` with the name x
+- Better search experience
+  - Project name and path validation
+  - The ability to point to a `project` by name instead of ID
+  - The ability to automatically create relevant orgs when creating projects
+    - if exists, use it; otherwise create a new one
+- The ability to set up a default backend during startup
+- Automatically lock the `stack` when other operations are in progress
+  - Gating mechanism for proceeding into the next stage in lifecycle (generate -> preview -> apply)
+- Automatically generate swag docs
+
+## Next up...
+
+These are what's considered interesting in the next few months for Kusion server to work on...
+
+### AuthN and AuthZ
+- This will likely be integration with open-source IAM solutions
+- More ways to AuthN and AuthZ than JWT
+- Support organizational roles and role assignments
+
+### Resource Graphs
+- Grant customized permissions on resources created by Kusion server
+- The ability to render a topological view of resources on said dimensions
+
+### GitOps
+- Manage automated GitOps triggers
+- Support master-worker architecture
+
+### Variables and Variable Sets
+- Support platform configurations that can set on any scopes: global, org, project, workspace, etc
+- Define priorities when multiple variables with the same key are present and may result in a scope conflict
+- Support more third-party secret management solutions for storing sensitive variables 
+
+### Audit trails and workflows
+- Create audit trails with subject, object and operation
+- Persist workflow executions, e.g. all the generate, preview, apply operations
+- The ability to insert custom stages in these workflow, e.g. approvals
+
+### Policies
+- Render-time (generate) policies and Execution-time (preview/apply) policy
+- Both built-in and customizable