Add design doc

Signed-off-by: Joe Nathan Abellard <[email protected]>

docs/proposals/karmada-operator/api-server-sidecard-containers/README.md

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+---
+title: Support for API Server Sidecar Containers in Karmada Operator
+authors:
+- "@jabellard"
+reviewers:
+- "@RainbowMango"
+approvers:
+- "@RainbowMango"
+
+creation-date: 2025-02-07
+
+---
+
+# Support for API Server Sidecar Containers in Karmada Operator
+
+## Summary
+
+This proposal aims to enhance the Karmada operator by introducing support for configuring additional containers  for the Karmada API server component.
+By allowing users to define additional containers, this feature unlocks new use cases such as integration with Key Management Service (KMS) plugins for configuring encryption at rest. 
+This enhancement provides greater flexibility while maintaining the declarative nature of the Karmada operator’s deployment model.
+
+## Motivation
+
+Currently, the Karmada operator provisions the API server as a standalone container within a pod. While the API server supports configurability via extra arguments, additional volumes, and volume mounts, it lacks built-in support for defining additional containers.
+By introducing explicit support for additional containers, this feature enables:
+
+- Seamless integration of auxiliary services such as KMS-based encryption providers.
+- Greater deployment flexibility by enabling users to extend the API server’s functionality without modifying the core operator logic.
+
+### Goals
+- Introduce support for configuring sidecar containers for the Karmada API server.
+
+### Non-Goals
+
+- Introduce changes beyond the scope of what is needed to support additional containers for the Karmada API server component.
+
+### API Changes
+
+```go
+// KarmadaAPIServer holds settings for the Karmada API server.
+type KarmadaAPIServer struct {
+    // CommonSettings holds common settings for the Karmada API server.
+    CommonSettings `json:",inline"`
+
+    // ExtraContainers specifies a list of additional containers to be deployed
+    // within the Karmada API server pod.
+    // This enables users to integrate auxiliary services such as KMS plugins for configuring encryption at rest.
+    // +optional
+    ExtraContainers []corev1.Container `json:"extraContainers,omitempty"`
+}
+```
+
+### User Stories
+
+#### Story 1
+As an infrastructure engineer, I want to integrate with my organization's KMS to configure encryption at rest for managed Karmada control planes.
+
+
+## Design Details
+
+During the reconciliation process, the Karmada operator will:
+
+- Check if `ExtraContainers` is specified in the KarmadaAPIServer spec.
+- If specified:
+  - Append the defined containers to the API server pod specification.
+- If not specified:
+  - Maintain the current deployment behavior with only the API server container.